Chop the onions and slice mushrooms 1/4-1/3 inch (5-8 mm) thick.
Sautée the onions in oil over medium heat until golden, for about 8-10 minutes. Lower the heat, add 4 tbsp (60 ml) water and let it simmer.
Cook the onions for about 30 minutes over low heat, adding 4 tbsp water at a time. (You can skip this step if you have no time, but this will create the nice and thick sauce for the onions.)
Take off the heat, add paprika and stir.
Add sliced mushrooms, salt and pepper, stir and let it simmer for 5-10 minutes without adding any liquid.
Mix sour cream with flour in a bowl, add 4-5 tbsp of mushroom sauce to avoid lumps.
Pour the sour cream mixture over the mixture, stir and let it cook for 2 minutes so that the flour thickens your mushroom goulash.