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Make the sauce: Heat the olive oil in a large wide pot over medium-high heat. Add the garlic and cook until golden brown, about 2 minutes. Add the tomatoes, oregano, red pepper flakes, 1/2 teaspoon salt and a few grinds of pepper. Bring to a boil, then reduce the heat to a simmer. Cook, stirring, until thickened, 15 to 20 minutes; season with salt. (You should have about 4 heaping cups of sauce.)
Make the stuffed shells: Preheat the oven to 375˚ F. Bring a large pot of salted water to a boil. Add the pasta shells and cook as the label directs for baking, about 9 minutes. Rinse under cold water to stop the cooking. Spread out on a baking sheet.
Heat the olive oil in a large skillet over medium-high heat. Add the spinach and cook, tossing occasionally, until wilted, 2 to 3 minutes. Remove to a large plate lined with a kitchen towel. Squeeze to remove any excess liquid, then finely chop.
Put the spinach in a large bowl and add the ricotta, mozzarella, parsley, Parmesan, lemon zest, beaten egg, 1/2 teaspoon salt and 1 teaspoon pepper. Stir until combined.
To assemble, spread about half of the sauce in the bottom of a 10-by-14-inch baking dish. Evenly fill the shells with the spinach-ricotta mixture using a spoon, or pipe it in using a pastry bag. Arrange the shells side by side, open-side up, in the baking dish on top of the sauce, then top with the remaining sauce.
Cover the dish with foil and bake until heated through and the sauce is simmering around the edges, about 30 minutes. Remove the foil and bake until any shells or cheese poking through the sauce are lightly browned in spots, about 10 more minutes. Let sit 10 minutes. Top with more Parmesan and red pepper flakes.