In a medium saucepan, cook the onion in the 4 teaspoons butter or margarine until tender, but not brown. Stir in the flour and the pepper. Add the milk all at once. Cook and stir until the mixture is slightly thickened and bubbly. Remove from heat. Add the Emmentaler cheese, stirring until melted (the mixture may appear slightly curdled). Stir in the cooked macaroni. Transfer the mixture to a 1-1/2-quart casserole.