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.
Bake in the preheated oven 15 to 20 minutes, or until the filling is golden brown.
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