In a skillet, heat olive oil over medium heat. Add chopped onion and garlic, sauté until softened, about 3-4 minutes.
Add zucchini, bell pepper, and mushrooms to the skillet. Cook for another 5-7 minutes until vegetables are tender. Stir in chopped spinach and cook until wilted. Season with salt, pepper, oregano, and basil. Remove from heat.
In a bowl, mix ricotta cheese with the egg until well combined.
Spread a thin layer of marinara sauce on the bottom of the slow cooker. Place 3 lasagna noodles on top.
Spread half of the ricotta mixture over the noodles, followed by half of the sautéed vegetables, and a third of the mozzarella cheese. Repeat the layers: marinara sauce, noodles, ricotta mixture, vegetables, and mozzarella. Top with the remaining noodles, marinara sauce, and the rest of the mozzarella and Parmesan cheese.
Cover and cook on low for 4-6 hours or on high for 2-3 hours, until the noodles are tender and the cheese is melted.
Let the lasagna sit for 10-15 minutes before slicing and serving.