1 jar (24 to 26 ounces) marinara sauce
1 ¼ cups water
1 package (24 ounces) frozen beef or cheese ravioli
3 to 4 cups shredded mozzarella cheese
1 box (10 ounces) frozen chopped spinach, thawed and squeezed dry
1/4 cup grated Parmesan cheese
Mix marinara sauce and water together in a large bowl. Spoon half the sauce into a 9 x 13 casserole dish that has been sprayed with cooking spray. Top with half of the ravioli, mozzarella cheese, and spinach. Repeat layer. Top with Parmesan cheese.
Bake, uncovered, at 350° for 25 to 40 minutes or hot and bubbly.