Melt butter in medium saucepan on medium-high heat. Add onions; cook 3 min., stirring occasionally. Stir in flour; cook 1 min., stirring constantly. Add milk and red pepper; stir until blended. Bring just to boil, stirring constantly. Simmer on medium-low heat 1 to 2 min. or until thickened, stirring constantly. Remove from heat.