Preheat oven to 400° F. Lightly grease 13x9 pan and set aside.
Fill a large pot roughly halfway and bring to a boil. Cook jumbo shells as directed on packaging. Drain well.
In a large skillet, heat olive oil at medium heat. Add chopped onion and garlic and cook for 2 minutes, stirring regularly.
Add in chopped red pepper and mushrooms. Cook about 2 to 3 additional minutes, stirring regularly.
Add in chopped spinach and mix until spinach has wilted in the pan. Remove from heat.
In a large bowl, mix ricotta cheese, Parmesan cheese, and your vegetable mixture.
To arrange your shells: Pour 1 cup of your favorite spaghetti sauce on the bottom of your 13x9 pan. Add 1 Tablespoon of water and mix to thin our your sauce. Stuff each jumbo shell with roughly 1 heaping Tablespoon of your ricotta cheese mixture and place open side up in your pan.
Cover shells with remaining 1 cup of your spaghetti sauce and top with Mozzarella cheese.
Cover and cook for 30 minutes, or until cheese has melted. Serve hot with your favorite side salad and bread. Enjoy!