Simple & Scrumptious Stuffed Shells

(From Pasta Primo Vino 2007)

1 package (12 ounces) jumbo pasta shells, cooked and drained
4 cups (16 ounces) shredded mozzarella cheese
1 container (15 ounces) ricotta cheese
3 heaping tablespoons grated parmesan cheese
1 package (10 ounces) frozen chopped spinach, thawed, drained, and moisture lightly squeezed out
1 jar (28 ounces) spaghetti sauce

Preheat oven to 350 degrees.
Combine cheeses in a large bowl. Break up spinach and add to bowl. Combine all. Stuff into shells. Place shells in lightly greased 13 x 9 baking pan. Pour spaghetti sauce over shells. Cover and bake for 30 minutes or until heated through.