Sauce Mornay (Cheese Sauce) is usually used for the base of a cheese soufflé or gratin. Classic recipes use half Gruyère and Parmesean. Today it is often used with many other cheeses. A combination of two well-aged cheddars works amazingly. Blue cheeses also work very well, but please choose genuine Roquefort, gorgonzola, stilton, etc.
For Sauce Mornay add four ounces of cheese per quart of Béchamel. Stir the sauce just long enough for the cheese to melt.