Heat oil in a large skillet over medium heat. Add garlic, onion, bell peppers, mushrooms, and spinach; cook and stir until vegetables are tender, about 10 minutes.
Stir diced tomatoes, Italian seasoning, salt, and pepper into the vegetable mixture; cook until heated through, about 5 minutes.
Bring a large pot of lightly salted water to a boil. Add lasagna noodles and cook for 8 to 10 minutes or until al dente; drain.
Spread 1/2 cup of the vegetable mixture into the bottom of a 9x13-inch baking dish. Top with a single layer of cooked lasagna noodles. Spread 1/3 of the ricotta cheese over the noodles. Sprinkle 1/2 cup of the mozzarella cheese over the ricotta cheese. Top with 1/2 cup of the vegetable mixture. Continue layering in this order until all ingredients are used. The top layer should be the mozzarella cheese.
Cover baking dish with aluminum foil.
Bake in preheated oven for 25 minutes. Uncover and bake an additional 25 minutes. Let stand 10 minutes before serving.
Interesting Facts
Lasagna originated in Naples, Italy.
Lasagna can be made with a variety of ingredients, from vegetables to meat to seafood.
Lasagna is a great dish to make ahead of time and freeze for a later meal.