2 lbs mushrooms
6 T butter
8 oz cream cheese
1/2 c crumbled Bleu Cheese
2 T ch onion

Remove mushroom stems.  Chop up enough to make 1/2 c.  Cook/Saute stems in butter for a bit.  Combine cheeses, add stems and onions.  Brown backs and tops of caps a little in the butter.  Put cheese mixture into caps. (Generally a good idea to make to this point and then take them to wherever they will be eaten before broiling.)  Broil until golden brown.
I never really follow the ratios here, especially the cream cheese to other cheese one.  Works more as “mix crumbled other cheese into cream cheese until you think it looks right.”  I also always have more goo than mushrooms to put the goo in, but it makes really tasty sandwiches, so I’ve never minded.  Particularly good using portabella, but you want to saute both sides of those fairly thoroughly before cheesing and broiling them.  Also works well with feta replacing the bleu.