Saute chopped onions, diced pepper and sliced mushrooms in 1/4 cup butter. Add to chopped clams, and season with salt, pepper and garlic.
Add the clam mixture to crushed Ritz crackers and 1/4 cup of melted butter until the mixture holds together but is not too greasy.
Stuff the clam shells with the mixture and bake for about 15 minutes at 350 degrees.
Garnish with a lemon wedge.