A little more tasteful side dish is au gratin mushrooms. My version is very simple.
8 small mushrooms (or 6 large)
Grated cheese to taste
Be sure to clean the mushrooms and remove the "feet".
Chop the feet together with the onion and fry in plenty of butter, season to taste.
Fry the caps on the top and bottom so you get a nice finish.
Put them in an ovenproof dish and fill the holes with the onion and mushroom mixture.
Top them with cheese and bake for about 10-15 minutes at 200C.