Remove stems from mushrooms; set caps aside. Finely chop stems. In a large skillet, saute chopped mushrooms and onion in butter for 5 minutes or until liquid has evaporated. Remove from the heat; set aside.
In a small bowl, combine the bread crumbs, pecans, Parmesan cheese, salt, basil and cayenne; add mushroom mixture. Stuff firmly into mushroom caps.
Place in a greased 15x10x1-in. baking pan. Bake, uncovered, at 400° for 15-18 minutes or until tender. Serve warm.