• Preheat the oven to 400ºF. Grease a 9 by 13 inch glass baking dish (or cast iron pan) with butter.
• Bring a large pot of salted water to a boil over high heat. Add the shells and cook until just barely tender, stirring occasionally, about 6 to 8 minutes. Drain the pasta and rinse with running cold water to stop the cooking. Set aside.

To make the Filling:

• Heat the oil over medium-high heat In a large skillet. Add the scallions and cook until soft, about 3 to 5 minutes. Add the garlic and cook for 1 minute longer. Sprinkle in the spinach or other greens, season with salt and pepper, and cook, stirring constantly, until wilted, about 2 to 5 minutes (kale may take longer). Allow the mixture to cool slightly. Place the ricotta, mozzarella and Parmesan cheeses, egg, basil, salt, and pepper in the bowl of a food processor. Add the cooled greens mixture and blend until smooth, about 15 pulses. Transfer the mixture to a bowl and stir in the peas. Season to taste with salt and pepper. Use a spoon to fill each shell with the mixture. Place the stuffed shells, filling side up, in the prepared baking dish.

To make the Fontina Sauce:

• Melt the butter in a medium heavy-bottomed saucepan set over medium heat. Add the flour and cook, stirring, for 2 minutes. Add the milk and cream and bring to a bare simmer, stirring often. Reduce heat to low and slowly add the fontina cheese, a handful at a time, stirring constantly, until the cheese is melted and the mixture is smooth. Remove the pan from the heat and stir in the nutmeg.
• Pour the sauce over the shells and sprinkle with the mozzarella and Pecorino cheese. Bake for 30 to 35 minutes until the sauce is bubbly and the top is golden. Cool for 5 minutes and sprinkle with basil before serving.