Clean mushrooms and carefully remove stems. Chop stems and onions finely (I use the food processor). Melt butter in a saute pan. Saute mushroom stems and onions over medium heat until soft but not browned and the stems have given up their juices. Mixture should be quite moist. Add the bread crumbs, only enough to absorb the liquid. Add the grated cheese and stir just until it has melted. Don't worry if the oil separates from the cheese.~
Stuff the mixture into the caps and sprinkle with a bit of salt (or you can salt after they come out of the oven...which is what I usually do).
Bake at 375 for 12-15 minutes or until the caps are nice and soft.