1. If White Sauce is not hot, heat sauce in medium-size microwave-safe bowl in microwave oven on high power for 1 to 2 minutes. Stir in the 2 cheeses, mustard and pepper until cheeses are melted. If cheeses don't melt, return sauce to microwave oven. Heat and repeat as above. Serve over assorted steamed vegetables.White Sauce:
2. For White Sauce, melt butter in heavy medium-size saucepan over medium heat. Whisk in flour; cook 1 minute. Increase heat to medium-high. Whisk in 1 cup milk; cook, whisking, until consistency of mashed potatoes, about 2 minutes. Add another 1 cup milk; cook, whisking, until thick and creamy with no lumps, about 2 minutes. Whisk in remaining 1 cup milk until combined.
3. Add onion, clove, bay leaf and nutmeg. Bring to boiling, whisking frequently. Reduce heat; simmer, whisking frequently, for 15 minutes; be careful sauce doesn't turn brown. Add salt and pepper. Stir; strain through wire-mesh sieve into bowl. Place buttered piece of waxed paper, butter side down, on surface of sauce. Let cool. Makes 3 cups.