Russian Monday: Mushrooms with Sour Cream Sauce - "Gribi v Smetane"

fresh mushrooms, I suggest baby portabella mushrooms for this recipe (or wild frozen mushrooms, I found them in Russian supermarket)

butter

olive oil

medium onion, peeled and diced

garlic gloves, finely chopped

salt and freshly ground pepper

all-purpose flour (or whole wheat flour)

sour cream

shredded cheese

chopped fresh dill to serve