In a skillet, saute onion and green pepper in 3 tablespoons butter until tender. Add flour, salt, mustard and cayenne; cook and stir until smooth and bubbly, about 2 minutes. Gradually add milk; bring to a boil. Boil for 2 minutes, stirring constantly. Stir in cheese until melted.
Remove from the heat; add hominy and olives if desired. Pour into a greased 1-1/2-qt. baking dish. Melt remaining butter and toss with bread crumbs; sprinkle over hominy mixture. Bake, uncovered, at 375° for 30 minutes or until golden.