Cook and stir onions and butter in skillet over low heat until onions are light brown. Sprinkle tuna, 1 cup of the cheese and the onions in baking dish.
Beat eggs, milk, baking mix, salt and pepper until well blended.
Pour into baking dish.
Bake 25 minutes. Top with remaining cheese to within 1 inch from edge. Bake 5 to 8 minutes longer or until knife inserted in center comes out clean. The pie can be done even though some melted cheese adheres to knife.