I recreated a Dutch Mustard Soup from a 19th-century cookbook, and its secret ingredient is something you would not expect.

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I’ve been making this Dutch Mustard Soup, mosterdsoep, for years and it still surprises me. I start with a sweet chopped onion and a spoonful of bold Dutch Zaanse mustard and somehow it becomes something sharp, silky and oddly comforting.

It’s not what you’d expect from a mustard soup, there is a brightness then a quiet creaminess that creeps up on you. I call it my Mustard Soup Recipe guilty pleasure, a lunch turned little celebration.

You might roll your eyes but trust me, this Dutch Mustard Soup will make you rethink what a simple soup can do.

Ingredients

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Onion: adds sweet savory base, contains fiber and vitamin C, helps mellow mustard heat.
Leek: mild slightly sweet onion family, gives body and gentle vegetal flavor.
Potato: thickens broth naturally with it’s starch, adds carbs and comforting heft.
Dijon and whole grain mustard: sharp tangy punch, adds depth and a little sodium.
Milk and cream make soup silky, add fat and calories, balances mustard bite.
Smoked sausage or ham: adds protein and smoky saltiness, optional but very satisfying.
Parsley: bright herb finish adds freshness and a little vitamin K.

Ingredient Quantities

2 tbsp unsalted butter

1 medium onion, chopped

1 leek white part only, sliced

2 celery stalks, sliced

1 medium potato about 200 g, peeled and diced

3 tbsp all purpose flour

1 litre (about 4 cups) chicken or vegetable stock

250 ml milk

100 ml heavy cream

3 tbsp Dutch Zaanse mustard or Dijon mustard

1 tbsp whole grain mustard

1 tsp sugar

1 bay leaf

Salt and freshly ground black pepper

150 g smoked sausage (rookworst) or diced cooked ham, optional

2 tbsp chopped fresh parsley for garnish

How to Make this

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1. Melt the butter in a large pot over medium heat; add the chopped onion, sliced leek white part and sliced celery with a pinch of salt, and sweat until soft and translucent, about 5 minutes, don’t let them brown.

2. Add the diced potato, stir for a minute to combine.

3. Sprinkle in the flour and stir constantly for about 2 minutes so the raw flour taste cooks out and you form a light roux.

4. Gradually whisk in the stock, then add the milk and the bay leaf; bring to a gentle simmer and cook until the potato is tender, about 12 to 15 minutes.

5. While it simmers, if using smoked sausage slice it and brown in a separate pan for extra flavor, or just set diced ham aside to add later.

6. Remove the bay leaf, then use an immersion blender to puree most of the soup until smooth but leave some texture, or transfer half at a time to a blender—be careful with hot liquid.

7. Return the soup to low heat and stir in the 3 tbsp Dutch Zaanse or Dijon mustard, the 1 tbsp whole grain mustard, 1 tsp sugar and the heavy cream; heat gently, do not boil, taste and season with salt and freshly ground black pepper.

8. Add the browned smoked sausage or diced ham to the pot and warm through for 2 to 3 minutes so the flavors marry.

9. Serve hot, sprinkled with the chopped fresh parsley. Adjust mustard or sugar to taste, it’s okay to tweak it a little.

Equipment Needed

1. Large heavy-bottomed pot for cooking the soup, dont try to use something too small.
2. Medium skillet for browning the smoked sausage or warming ham.
3. Immersion blender, or a regular blender to puree half the soup (careful with hot liquid).
4. Wooden spoon or heatproof spatula for stirring the roux and soup.
5. Whisk for blending in the flour and stock smoothly.
6. Chef’s knife for chopping onion, leek, celery and potato.
7. Cutting board, sturdy and big enough to work on.
8. Measuring cups and spoons for stock, milk, mustards and seasonings.
9. Ladle and soup bowls for serving, plus a small spoon for tasting.

FAQ

Can I make this soup vegetarian?

Yes. Use vegetable stock instead of chicken, skip the smoked sausage and add smoked tofu, seared mushrooms or a vegetarian sausage. If you miss the smokey note try a small pinch of smoked paprika or a tiny drop of liquid smoke, taste as you go.

How spicy will it be and can I make it milder?

The Dutch mustard gives a nice bite but the milk and cream mellow it. Start with the recipe amount of mustard then taste before serving, add more if you want it punchier. To make it milder add a little extra cream or a teaspoon of sugar till it suits you.

Can I freeze this soup?

You can but cream can change texture. For best results cool the soup, remove some to freeze without the cream, freeze up to 3 months. When reheating thaw slowly, add the cream and milk at the end and rewarm gently while whisking.

My soup is too thin or too thick what do I do?

Too thin simmer a bit longer uncovered to reduce it, or mash a few potato pieces or stir in a spoon of flour mixed with cold water. Too thick add a splash of stock or milk until you like the consistency.

Is rookworst already cooked and when do I add it?

Most smoked Dutch sausages are pre cooked so you only need to heat them through. Slice and add in the last 5 minutes of cooking so they warm without falling apart. If using raw sausage cook it first till done.

How long will leftovers keep and how should I reheat them?

Keep in the fridge for up to 3 days in a covered container. Reheat gently on the stove over low to medium heat, stir often. Dont let it boil hard once you added the cream, that helps prevent curdling.

Dutch Mustard Soup Recipe Substitutions and Variations

Unsalted butter: use olive oil, ghee, or vegan buttery spread — olive oil gives a brighter flavor, ghee adds a nutty note and vegan spread keeps it dairy free.
Chicken or vegetable stock: swap for low sodium stock, good quality bouillon dissolved in water, or plain water with a splash of soy sauce for extra umami.
Milk and heavy cream: use half and half, evaporated milk, or full fat coconut milk for a dairy free creamy finish (coconut will change the flavor a bit).
Smoked sausage (rookworst): replace with kielbasa, crispy bacon pieces, smoked tofu or diced seared mushrooms for a vegetarian smoky note.

Pro Tips

Sweat the onion leek and celery gently with a pinch of salt so they soften without browning, dont rush this step, it builds the base flavor. Use a lid sometimes and cook on medium low, stir now and then.

When you add the flour cook it until it smells toasty, whisk constantly and pour the stock in slowly so you dont get lumps. If lumps happen push the soup through a sieve or blitz it with the immersion blender.

Brown the smoked sausage in a separate pan for real caramelized flavor, and save a spoon of the browned fat to stir in at the end. That little bit of fat gives the soup more depth than just tossing in cold meat.

Tweak the mustard and sugar in small amounts and taste as you go, remember flavors mellow after resting so dont overdo it. If it gets too sharp a splash of cream or a small pinch of sugar calms it down fast.