Cook tortellini according to package instructions. Once cooked, drain and rinse with cold water. Set aside. TIP: I found it easy (and a timesaver) to cook the tortellini at the same time the zucchini/sauce mixture was cooking.
Heat olive oil in large skillet on medium high heat. Add diced zucchini and chopped onions, stir often, and cook on medium high heat for about 5 minutes (the veggies will be very lightly browned). Add the minced garlic to the skillet, stir to combine, and cook for another minute (be careful to stir and not let the garlic burn, or it will taste bitter).
Add baby spinach leaves, 1 cup (reserve the rest) of marinara sauce, red pepper flakes, salt and pepper to the skillet. Stir well, to combine. TIP: The spinach can be added in small batches... it looks like a LOT of spinach going in the skillet, but it will definitely wilt down! Once it has wilted, add more to skillet. Once all spinach has been added, cover the skillet, and simmer on low heat for 2-3 minutes, until the spinach has fully wilted. At this point, additional salt and pepper can be added, to suit your taste.
Layer the casserole this way: Spoon 1/2 cup of the reserved marinara sauce over bottom of dish. Add HALF of the cooked/drained tortellini on top of the sauce. Cover the tortellini with HALF of the veggie/sauce mixture. Cover that with the rest of the tortellini. Cover that with the rest of the veggie mixture. Pour remaining 1/2 cup of marinara sauce over top of veggies, then sprinkle with grated mozzarella and Parmesan cheese.
Cover the dish loosely with aluminum foil (try not to let the foil touch the top of the cheese, if possible). Bake in a 350 degree oven for 20 minutes, then remove foil and bake for 10 more minutes, or until cheese is melted and bubbly. Remove casserole from oven. Serve, and enjoy!
TIP: If desired, recipe can be made using FROZEN tortellini (packaged without sauce) in a SLOW COOKER, following same process as detailed above (except for pre-cooking the tortellini). Tortellini can be layered in while still frozen. Casserole should be cooked on LOW (covered) until tortellini are tender and fully heated through, (approximately 5 hours). I have not personally tried this, but Weight Watchers recipe utilized this cooking method.
Tortellini Marinara and Zucchini Bake https://www.thegratefulgirlcooks.com/tortellini-marinara-zucchini-bake/