World's Easiest Souffle

This is a favorite recipe from the American Egg Board.

Butter
Grated Parmesan cheese
4 eggs
4 ounces sharp Cheddar cheese, cubed
3 ounces cream cheese, cubed
1/3 cup milk, light cream or half-and-half
1/4 cup grated Parmesan cheese
1/2 teaspoon onion salt
1/2 teaspoon dry mustard

Butter bottom and sides of 1-quart souffl