Preheat oven to 375°F. Spray an 8x8-inch casserole dish with cooking spray.
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.
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.)
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.
Let rest for 10-15 minutes before serving. Top each serving with grated Parmesan, if desired.
Freezer Instructions
Freeze For Later: Spray an 8x8-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.
Notes/Tips
If using fresh tortellini, this will only take about 20-25 minutes to bake.
You can also make this with frozen cheese or beef ravioli, but you'll have to make it in three layers instead of two so that each ravioli layer is flat and they aren't overlapping. Then, bake covered for 45 minutes and uncovered for 15 minutes.