A mushroom soup with mascarpone, made without flour or potatoes and thickened with agar-agar.
Golden mushroom slices bring toasted notes, mascarpone makes the broth silky, and rosemary adds a resinous touch. This mushroom soup with mascarpone is a low-carb version, made without flour or potatoes and thickened with agar-agar. Ready in 15 minutes total, including cooking time, it makes 2 servings and contains 8 g of net carbs per serving. File it alongside my keto soups and salads.
What is mushroom soup with mascarpone?
The idea behind mushroom soup is simple: mushrooms take center stage, with no single fixed formula, as in some comforting Asian soups. Here, the slices remain visible, there is no blending, and mascarpone provides the creaminess. The low-carb version relies on leaving out flour and potatoes, without promising strict ketosis.
The ingredient list is short. Onion, garlic, and rosemary form the aromatic base, broth adds body, and tomato paste lends a reddish tint and subtle depth. Agar-agar gives the soup a little structure as it cools: the final texture stays smooth and uniform, without white flecks of mascarpone or oily patches, and the mushrooms retain some bite.
Recipe origins and key features
Mushroom soup appears in many home-cooking traditions, sometimes smooth and blended, sometimes more rustic with visible chunks. This recipe belongs to the second family, with specific choices: mascarpone, agar-agar, rosemary, and mushrooms seared separately.
These choices change the result. Flour and potatoes, often used for thickening, make way for agar-agar, a gelling agent derived from red algae. Mascarpone replaces the usual cream for a thicker result with a milkier flavor, while rosemary and tomato paste give it a woodsy, lightly roasted character and a pinkish color rather than a creamy white.
The key is still cooking the mushrooms separately: a large skillet, high heat, slices that are not overcrowded, and enough time for the water to evaporate so the edges become nicely golden.
The main ingredients
Cremini mushrooms are the base of the soup. With 250 g for 2 servings, they provide texture, umami, and a stronger flavor than white button mushrooms; slice them a little thicker than you want them, as they shrink during cooking.
Two small onions, or one very large one, provide an underlying sweetness, and 3 pressed garlic cloves, added once the onions are translucent, bring warmth and depth. A generous handful of fresh rosemary adds a pronounced fragrance, with 2 extra sprigs for garnish. The 2 teaspoons of agar-agar thicken mainly as the soup cools, so there is no need to add more while the soup still looks loose when hot.
The 350 ml of chicken broth or vegetable broth deglazes the base and carries the seasoning; the vegetable version remains vegetarian but not vegan because of the mascarpone. The 150 g of mascarpone adds richness without blending and should be incorporated away from a vigorous boil to prevent splitting. The 3 teaspoons of tomato paste, sautรฉed before adding the broth, provide depth, two pinches of salt and pepper adjust the seasoning, and two drizzles of oil help brown the mushrooms.
Technique tips and serving
Success comes down to two textures. Seared in a large skillet without overcrowding, the mushrooms brown rather than cook in their own juices, like intensely flavored braised mushrooms. The broth, meanwhile, does not thicken as it would with flour: agar-agar can firm up the soup in the refrigerator, and gentle heat with a little water brings it back to a looser texture when reheated.
The order is simple: translucent onions, then garlic and rosemary, then tomato paste heated through before adding the broth, a gentle simmer, agar-agar, mascarpone gently melted in, and seared mushrooms added at the end of cooking. Serve in bowls with a sprig of rosemary; a green salad balances the richness of the mascarpone, while bread and croutons are still options but are higher in carbs. For leftovers, cool the soup quickly and refrigerate in a sealed container. Per serving, allow 383 kcal and 8 g of net carbs, with a cautious range of 8 to 14 g depending on the weight of the onions.
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Mushroom Soup with Mascarpone
Ingredients
- 250 g cremini mushrooms
- 2 small onions or 1 very large onion
- 3 cloves garlic
- 1 generous handful of rosemary fresh (plus 2 sprigs for garnish)
- 2 teaspoons agar-agar
- 350 ml chicken stock or vegetable stock
- 150 g mascarpone
- 3 teaspoons tomato paste
- 2 pinches Celtic sea salt
- 2 pinches black pepper
- 2 generous splashes avocado oil, mild olive oil or high-oleic sunflower oil
Instructions
- Finely slice the onion. Sautรฉ it in the pot with a generous splash of oil until translucent.2 small onions, 2 generous splashes avocado oil, mild olive oil or high-oleic sunflower oil
- Crush the garlic. Finely chop or snip the rosemary.3 cloves garlic, 1 generous handful of rosemary
- Once the onions are fully translucent, add the garlic. Then add the rosemary.
- Meanwhile, cut the mushrooms into slices slightly thicker than you want them to be at the end. They will shrink further. I generally get 4 slices from a slightly smaller mushroom and 5 from a slightly larger one.250 g cremini mushrooms
- I prefer to sautรฉ the mushrooms in a separate frying pan. Over high heat, with a generous splash of oil. Let them brown well on all sides. And only add them to the soup at the very end. This gives them a lovely caramelized flavor, and they lose less water. If you are short on space, sautรฉ the mushrooms with the onion in the pot.
- Briefly sautรฉ the tomato paste with the rest of the ingredients, if using.3 teaspoons tomato paste
- Deglaze the pot of onions with the stock. Heat through. Let it simmer gently. Add the agar-agar.350 ml chicken stock, 2 teaspoons agar-agar, 2 pinches Celtic sea salt, 2 pinches black pepper
- Add the mascarpone. Stir until it has melted into the soup.150 g mascarpone
- Add the mushrooms a minute or two before serving.
- Pour into bowls and garnish with a sprig of rosemary.
