I got this recipe from my very best very vegan friend (you know who you are Patty). I have adapted it slightly (because where I shop I don't buy the vegan type butter and such, and Patty forgives me:). They are oh so good, and great for that vegetarian in your life!
bag Frozen Spinach
2-4 Tablespoons butter/margarine
1/4 C Chopped onions
a wee bit of garlic powder
bread crumbs (I use the Italian flavored kind)
Remove stems from mushrooms and dice (stems) very fine. Set aside mushroom tops. Sautee the finely diced mushroom stems and onion in the butter. When onions are transparent, add 1/2 bag of frozen spinach. (These amounts are based on how many mushrooms you make. I usually make the whole box the mushrooms come in. If you did two you would need to use more spinach!) Give that garlic powder a couple of shakes over your concotion. Let that spianch thaw and cook with the sauteed mushrooms and onions for a couple minutes. Now it's time to add the bread crumbs. (If you use unflavored bread crumbs, your likely to want more garlic and some other spices) Add about a 1/4 cup of the bread crumbs at a time. You want this to make a nice "stuffing" consistancy. Once your happy with your stuffing, it's time to stuff it! Fill your mushrooms to the brim with your delicious stuffing. Place the stuffed mushrooms on a pan and bake at 350 degrees for about 20 minutes. This is also a great dish that you can make the night before and bake right before company arrives! You can also add a bit of motzerella cheese to top it off for added flavor (although it doesn't need it).... and the left over stuffing (enjoy it... Yummm)