Preheat the oven - Preheat the oven to 375°F (190°C). - Make sure to preheat the oven so that it is ready to bake the lasagna when it's assembled.

Boil the lasagna noodles - Bring a large pot of salted water to a boil. Cook the lasagna noodles according to the package directions. Drain the noodles and rinse them with cold water. - Be careful when handling the hot water and noodles. It helps to add a little bit of oil to the boiling water to keep the noodles from sticking together.

Make the tomato sauce - In a skillet over medium heat, sauté the chopped onions and minced garlic in olive oil for about 3 minutes until fragrant. Add sliced mushrooms and continue sautéing for 5 more minutes until the mushrooms have released their liquid. Pour in the crushed tomatoes and tomato paste, and add fresh chopped basil, salt, and black pepper. Reduce the heat, and let the sauce simmer for 10 minutes until thickened. - Be careful not to burn the garlic or onions. You can use a can of diced tomatoes instead of crushed tomatoes but make sure to puree it in a blender or food processor before using.

Prepare the sweet potatoes - Slice the sweet potatoes lengthwise into thin pieces using a sharp knife or mandoline. If the sweet potato is large, cut it in half first. Rinse them under cold water and pat dry with a towel. - If you don't want to slice the sweet potato, you can cube or mash it instead. However, ensure even cutting and slicing to ensure uniform cooking.

Assemble the lasagna - In a large mixing bowl, stir together ricotta cheese, baby spinach, and grated parmesan cheese. Spread a layer of tomato sauce on the bottom of the lasagna dish. Layer the lasagna noodles over the sauce, slightly overlapping them. Top the lasagna noodles with a layer of sweet potatoes, followed by a layer of ricotta cheese mixture. Repeat layers, ending with a layer of lasagna noodles on top. Spread the remaining tomato sauce over the top layer of lasagna noodles and sprinkle shredded mozzarella cheese over the top. - Be generous in the cheese layers. Ensure not to overstuff with vegetables as it may prevent even cooking and layers

Bake the lasagna - Cover the lasagna dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 20-25 minutes until the cheese is melted and bubbly. Let the lasagna cool for a few minutes before slicing and serving. - Make sure you tent the foil over the lasagna, so that the cheese doesn't stick to it.

Serve the lasagna - Serve the vegetarian lasagna hot, garnished with fresh chopped herbs if desired. Enjoy! - This lasagna is best served fresh, but it can be stored in the refrigerator for up to 3 days, stored in an airtight container.