Wash the mushrooms and tomatoes. Cut the mushrooms into quarters. (If they are quite large also cut smaller).
Peel and finely dice the onion. Heat the keto margarine in a pan, first sweat the onions until translucent and then add the mushrooms and fry until they take on some color. Season with salt and pepper.
Clean the cauliflower and cut into florets. In a pot, bring salted water to a boil and cook the cauliflower until al dente. Drain the cauliflower but put a good half liter of the cauliflower water in a measuring cup.