Prepare garlic tomato sauce by adding olive oil to a large skillet over medium heat. Add onions and garlic, and sauté until softened and just beginning to brown.
Add in the diced tomatoes, tomato paste, water, and seasonings. Stir together, and then simmer for about 15 minutes to allow sauce to reduce. Set aside.
Layer the sauce, butternut noodles, cheese mixture, mozzarella cheese, and spinach. Repeat 3 layers!
When the lasagna is finished baking, top with reserved garlic tomato sauce and mozzarella cheese. Increase the temperature of the oven to 425ºF, and return to the the lasagna to the oven for 15 minutes, until the top is browning and bubbly.