To prepare sauce, melt butter in a medium saucepan over medium-low heat. Whisk in flour until combined. Cook about 2 minutes. Whisk in milk, salt, pepper and nutmeg. Add sour cream. Bring mixture to a slow simmer. (Do not boil.) The mixture will begin to thicken. Remove from heat. Add cheese and stir until smooth.
To make pizza, preheat oven to 425°F.
Roll pizza dough into a 14-inch round or square. Place on baking sheet or pizza stone.
Spread Mornay sauce on dough and top with remaining ingredients in the order listed.
Bake 7 to 11 minutes or until puffy and golden brown.