Preheat oven to 400 degrees F (200 degrees C).

Place ground beef in a large, deep skillet. Cook over medium high heat until evenly brown. Drain, crumble and set aside.

In a medium saucepan over medium heat, melt the margarine and stir in the mushroom stems, green bell peppers, garlic, parsley, and basil.

In a large bowl, mix the ground beef, mushroom stem mixture, dry bread crumbs, soft bread crumbs, and Cheddar cheese.

Place the mushroom caps, upside down, on a large baking pan. Generously stuff each cap with the mixture.