2 tbsp. butter
2 tbsp. flour
1/4 tsp. salt
Dash cayenne
1 tsp. minced onion
1 1/2 c. milk
1/2 c. Gruyere cheese, diced
2 tbsp. freshly grated Parmesan

Melt butter. Blend in flour until smooth. Add salt, cayenne, and onion. Slowly add milk and cheeses. Stir constantly until sauce thickens. Yields 1 3/4 cups.