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1
Preheat the oven to 400F.
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2
Bring a large pot of water to a boil for the pasta.
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3
Salt the water and cook the shells for 2 to 3 minutes less than the package directions.
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4
Heads up: You will need to reserve 1/2 cup starchy cooking water just before draining the pasta.
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5
Meanwhile, heat a medium-size pot or high-sided skillet over medium heat with the EVOO.
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6
Add the onions and garlic and saute for 3 to 4 minutes.
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7
Add the spinach, pulling it apart as you add it, and season with a little freshly grated nutmeg, salt, and pepper.
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8
When the spinach mixture is warm through, transfer it to a small dish and reserve.
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9
Return the pot to the heat, add the butter, and melt.
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10
Add the tomatoes and break up with a spoon.
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11
Season with salt and pepper, add the basil leaves, and cook to thicken the sauce, 7 to 8 minutes.
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12
Drain the pasta and return the pasta to the hot pot.
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13
To the pasta, add the spinach, reserved starchy water, ricotta cheese, and half of the Parmigiano-Reggiano.
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14
Adjust the salt and pepper and transfer to a baking dish.
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15
Top the pasta mixture with the tomato sauce, mozzarella, and remaining Parmigiano.
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16
Bake until the cheeses are melted and bubbly, 6 to 8 minutes.
