1/4 tsp. cayenne pepper (more or less depending on how hot you like it)
Make a Roux with butter and flour. Add other ingredients and simmer for 5 min. or longer till thickens. Put in casserole dish. Sprinkle top with parmesan cheese then broil till brown. If it is not thick enough for you - next time use 3-4 TBSP flour in roux.