Peel and cut the onion into small pieces and start to saut it on 2tbsp oil in a pan. Salt to taste. Use a slow fire to make them sweet naturally, stir so they dont burn.
Peel the mushrooms and cut them into slices, stems included. Take the caramelized onions out of the pan, leave the onion-oil in the pan, and add 2 more tbsp oil. Fry the mushrooms on slow fire, stir sometimes
When mushrooms are halfway cooked,addd the mustard, stir well, also add back the onions. Use slow fire, stir often. And salt to taste. First juice of mushrooms will come out, later it evaporates and only oil remains, at this time stir often and make sure the mushrooms get a nice brownish color
