Preheat the oven to 400°F. Bring a large pot of salted water to a boil and cook the lasagna noodles according to the package directions until al dente. Drain, then lay the noodles flat on a lightly oiled baking sheet so they don't stick together.
Heat the olive oil in a large non-stick skillet over medium heat. Add the onion and cook for 3-4 minutes, until softened. Add the garlic and spinach and cook for 2-3 minutes, stirring, until the spinach is wilted and most of the liquid has cooked off. Set aside.
Lightly grease a 9x13-inch baking pan with non-stick cooking spray. Spread 1 cup of marinara sauce over the bottom of the pan.
Lay down a single layer of lasagna noodles, overlapping them slightly. Top with a layer of the spinach filling, then another layer of sauce and noodles, then dollops of vegan ricotta and a sprinkle of vegan mozzarella. Repeat the layers until all the noodles are used. Finish with the remaining marinara sauce, a final sprinkle of vegan mozzarella and the sliced tomatoes, if using.
Bake for 20-25 minutes, until the sauce is bubbling and the cheese is melted. Let the lasagna rest for 10 minutes before slicing and serving.
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Notes
No-boil noodles: Skip step 1's boiling, add an extra ½ to 1 cup of marinara so every noodle is covered, and bake covered with foil for the first 15 minutes.Make ahead: Assemble the lasagna up to 1 day ahead, cover and refrigerate, then add 5-10 minutes to the baking time.Storage: Refrigerate leftovers for up to 4 days or freeze for up to 3 months.
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