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Place a rack in top-third of oven; preheat to 425°. Bring a large pot of generously salted water to a boil.
Meanwhile, get some prep work out of the way. Peel 1 large onion, then finely chop. Set aside on cutting board. Crush 6 garlic cloves with the side of a chef’s knife, then peel and coarsely chop.
Heat ¼ cup extra-virgin oil in a 12" ovenproof skillet over medium. Cook onion and garlic, stirring occasionally, until onion is beginning to brown and soften, about 5 minutes.
Add 3 14.5-oz. cans crushed tomatoes, 1 tsp. dried oregano, ¾ tsp. crushed red pepper flakes, 3 whole basil sprigs, and 1 tsp. salt. Cook, stirring occasionally, until sauce is reduced and slightly thickened and flavors have melded, about 15 minutes. Remove from heat and pluck out basil sprigs.
Meanwhile, place 20 oz. thawed spinach on a large clean kitchen towel and wring it out over the sink until no more water can be squeezed out. If you don't have any clean kitchen towels, you can also squeeze water out of the spinach by hand one small handful at a time, or press it in a fine-mesh sieve. Whatever you do, just don't skip this step—waterlogged spinach will make the finished dish too soupy.
Mix spinach, 2 cups ricotta, 1 tsp. freshly ground black pepper, 3 oz. grated Parmesan, and remaining 1 tsp. salt in a medium bowl to combine.
Cook 10 oz. lasagna noodles in boiling salted water, stirring occasionally and separating with a spatula to keep them from sticking, 4 minutes (they'll be very al dente at this point, but will finish cooking in oven). Drain noodles and leave in colander until ready to assemble.
Using a ladle, transfer 3 cups sauce to a medium bowl, leaving just a thin layer of sauce behind. Spread to coat bottom of pan.
Arrange noodles in a single layer over sauce in pan, breaking up a few sheets to fit and cover the whole surface. Top noodles with one-third of tomato sauce, spreading evenly with a spatula. Using a spoon, dollop half of the ricotta mixture in small mounds all over sauce. Sprinkle one-third of shredded mozzarella over ricotta mixture.
Repeat layering noodles, one-third of sauce, remaining ricotta mixture, and one-third of mozzarella one more time. Top mozzarella with another layer of noodles. Spoon any remaining sauce over, then top with remaining mozzarella.
Bake lasagna until browned and bubbling, 15–20 minutes. Sprinkle some Parmesan over, then garnish with remaining basil leaves.