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1
Cook shells in a large pot of boiling salted water, stirring occasionally, until very al dente, about 9 minutes; drain. Run under cold water to stop the cooking; drain again.
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2
Meanwhile, heat oil in a large skillet over high. Add mushrooms and cook, stirring occasionally, until they release juices, then are dry again and nicely browned, 5-6 minutes; season with black pepper and 1 tsp. salt. Reduce heat to medium, add wine, and cook, stirring, until reduced by half, 1-2 minutes. Add spinach, cover, and cook until beginning to wilt, 1-2 minutes. Uncover and continue to cook, stirring occasionally, until spinach is completely wilted and most of the liquid is evaporated, 2-4 minutes more. Transfer mushroom mixture to a large bowl; reserve skillet.
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3
Cook garlic and butter in reserved skillet over medium-high heat, stirring occasionally, until garlic is fragrant and beginning to brown, 2-3 minutes. Add marinara sauce and red pepper and bring to a simmer over low heat. Cook, stirring occasionally, until warmed through, 6-8 minutes.
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4
While sauce cooks, add ricotta, 3 oz. Parmesan, 2 Tbsp. oregano, and remaining 1/2 tsp. salt to mushroom mixture and stir to combine. Spoon about 2 Tbsp. ricotta mixture into each shell. The shell should be filled to capacity but not overstuffed.
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5
Nestle stuffed shells into hot sauce in skillet. Cover and cook over medium heat until shells are warmed through, 4-6 minutes. Remove from heat and let sit 5 minutes. Sprinkle with Parmesan and remaining 1 Tbsp. oregano.
