Add the onions, and cook 4-5 minutes until they are soft and golden.
Add the garlic, and sauté, being careful not to burn the garlic.
Add the tomato paste and stir well. Add the chopped tomatoes, including the juice in case you are using canned tomatoes.
Bring to a low simmer, cover, and cook for 25-30 minutes.
Finally, remove from the heat, and add the fresh basil and spinach. Stir well.
Adjust the seasoning if necessary.
Arrange the zucchini slices in a single layer on a baking sheet coated with cooking oil spray. Broil for 5-8 minutes. Remove from the oven. Wait about 5 minutes to remove any excess moisture with paper towels if necessary. (This part is very important to avoid the lasagna becoming too soupy.)
Preheat the oven to 375°F.
Place some of the ricotta cheese mixture on the zucchini slices, and top with some mozzarella cheese. Repeat the layers until all your ingredients are used.
Cover the casserole dish with aluminum foil, and bake for 30 minutes. Uncover, and cook an additional 10-15 minutes.
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