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Preheat oven to 375 F.
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Cook pasta, drain, set aside.
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Heat oil in 12 skillet oven medium-high heat. Add the sausage, brown, breaking up as it cooks, 10 to 12 minutes. Remove from skillet.
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Place tomatoes in a bowl with juice, crush into chunky pieces.
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In a large bowl combine ricotta, Parmigano-Reggiano, mozzarella, egg, spinach, garlic, nutmeg, 1 tsp salt and pepper. Mix with a spoon until smooth. Fold in cooked pasta.
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Grease 3 ½ qt cast-iron Dutch oven with oil and spread about ⅓ of tomatoes across the bottom. Top with ½ of the pasta mixture spread evenly. Distribute half of the cooked sausage. Repeat with ½ of the remaining sauce and the rest of the pasta mix and sausage. Top with remaining tomato sauce.
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Bake uncovered 20 minutes. Top with layer of mozzarella slices and bake for 10 minutes. Place under broiler for 2 minutes if cheese is not browned.
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Cool 10 minutes before serving.