Preheat oven to 375 degrees F. Lightly coat a 9x13" casserole dish with non-stick spray and set aside.
Bring a large sauce pan to medium-high heat and add in oil. Cook sausage, breaking up until browned and cooked through. Add in zucchini, mushrooms, garlic, Italian seasoning and onion powder. Cook until veggies are tender. Scrape any browned bits off the bottom of pan as necessary.
Add in the pasta sauce and ¼ cup of water. Bring sauce mixture to a simmer for just 3 to 5 minutes.
Layer the casserole with 1 cup of sauce on the bottom. Add ½ of the frozen ravioli in an even layer, top with ½ of the remaining sauce and ½ of the mozzarella cheese. Repeat layers with the remaining ravioli, sauce and cheese. Cover tightly with foil and bake for 60 minutes.
Uncover casserole and continue baking for an additional 15 to 20 minutes, until the casserole is bubbly throughout and cheese is melted in middle. If desired, turn on the top broiler and brown cheese slightly. Let the casserole rest for about 10 minutes before serving. Enjoy!
If your casserole is frozen, let defrost in fridge for 24 hours before baking as directed. BAKE FROM FROZEN? You can bake this casserole straight from the freezer as long as it's in a metal or foil baking dish. Bake covered at 375 degrees F. for about 90 minutes (or until center is hot). Remove foil and continue baking for an additional 15 to 20 minutes until heated throughout.