Macaroni and Cheese

1/4 cup butter
1/2 cup chopped onion
1/4 cup flour
2 1/2 cups milk
3 cups shredded sharp Cheddar cheese
1 cup grated Parmesan
2 Tsp Dijon mustard
1 1/2 tsp each salt and pepper
1 lb pasta, cooked

In a sauce pan, melt butter and fry onion until soft. Add in flour and whisk until throughly blended. Add in the milk slowly, whisking as it is added. When the mixture bubbles and thickens slightly, remove from heat. Stir in 2 cups Cheddar, the Parmesan, mustard, and salt and pepper. Add Tabasco if desired. Stir until cheese is melted and then fold into the drained pasta. Top with remaining 1 cup Cheddar. Pour into a greased 9X13 pan. Bake at 375 for 30 minutes.