Make It Now:

1. Preheat oven to 375°F. Spray an 8×8 inch casserole dish with cooking spray.
2. Spread thin layer of marinara sauce into bottom of baking dish. Arrange half of the frozen tortellini over the sauce, trying to get them in a single layer as much as possible. Top with 1/2 of the chopped spinach (and pepperoni), 1/2 of the sauce (use the back of a spoon to move the sauce around and make sure it covers all the tortellini), and 1 cup of mozzarella cheese. Repeat layers in Step 2, ending with the last of the cheese on top. (Freezing instructions begin here.)
3. Tent lightly with a layer of foil; try to avoid touching the cheese. (Tip: Spray with cooking spray on the inside of the foil to help prevent the cheese from sticking.)
4. Bake for about 40-45 minutes until bubbly on the edges (if using fresh tortellini, bake 20-25 minutes). Take off the foil and bake about 10-15 minutes more, or until golden on top and hot throughout.
5. Let rest for 10-15 minutes before serving. Top each serving with grated Parmesan, if desired.

Freeze For Later: Spray an 8×8 inch casserole dish with cooking spray. Follow Step 2. Cover the dish with foil and a few layers of plastic wrap or foil. Label and freeze.

Prepare From Frozen: Thaw in the refrigerator. Preheat the oven to 375°F. Bake for 20-25 minutes covered in foil and then another 10-15 minutes uncovered.