Combine the cheeses, egg, parsley, salt, and pepper in a large bowl.
Stir together until completely combined.
Cook pasta.
Ensure that pasta water has olive oil to keep the shells from sticking to one another.
Stir frequently while cooking.
Ladle 2 cups of sauce across the bottom of a 9x13 pan.
Stuff each shell with roughly 1-2 tablespoons of filling.
Arrange the stuffed shells carefully in the pan.
Bake stuffed shells at 375F for 30 minutes, or until the cheese is browned and the sauce is bubbling.
Repeat with a second pan, or in a freezer tin.
These can be prepped in an aluminum freezer pan and frozen for up to six months. Thaw for 12 hours before baking.