In a flat-based wok, cast iron skillet or large pan, gently fry the onion on low-medium heat in a tablespoon of butter and tablespoon of olive oil for five minutes or so until they soften, brown and caramelise, then set aside.
Add the finely chopped garlic cloves, a splash of oil if needed, and fry for a minute on low heat. Add another tablespoon each of butter and olive oil, the mushrooms, salt and black pepper, stir the mushrooms so they’re covered in the butter, oil and seasoning, turn the heat up to high and sauté the mushrooms until brown and soft.
Turn the heat down to low, return the onions, add the cream and sour cream, allspice, paprika, mustard, and fish sauce, stir to combine well, then gently simmer for ten minutes or so. If the sauce has not thickened enough, turn the heat up to medium-high or even high for a few minutes or so to reduce.
Taste and adjust the seasoning if necessary to suit your taste, adding a little more salt, pepper, spices, and even another spoon of fish sauce if you like. Simmer for a few more minutes so the flavours meld together and taste again.
Dish out into individual bowls, add a dollop of sour cream, garnish with plenty of fresh dill, and serve with bowls of gherkins and additional sour cream, and sides of your choice – buckwheat kasha, pasta, rice, crispy shoestring fries, and mashed potato all work. A Russian garden salad is a must.