Spinach and Ricotta Stuffed Shells

Steps

1. Make the marinara: in a saucepan, warm 3 tablespoons of the olive oil over medium-low heat with 3 sliced garlic cloves and the red pepper flakes for 2 minutes, until the garlic is barely golden. Add the canned tomatoes, crushing them with your hand, the basil stems, and 1 teaspoon of the fine salt. Simmer uncovered for 25 minutes, stirring now and then, until the sauce thickens and the oil rises to the surface, then remove the stems.
2. Meanwhile, bring a large pot of water to a boil with the coarse salt. Add the pasta shells and cook them 3 minutes less than the package says: they will finish cooking in the oven as they drink up the sauce. Drain gently and spread them on a sheet pan oiled with 1 tablespoon of the olive oil, open side down, so they do not stick together.
3. Thaw the frozen spinach in a skillet for a few minutes, let it cool slightly, then wrap it in a clean kitchen towel and wring it as hard as you can over the sink, until not one more drop comes out. You should be left with a compact ball of about 250 g (about 1 cup). Chop it finely.
4. Preheat the oven to 375°F (190°C). Grate the mozzarella and the hard cheese. In a bowl, mix the ricotta, chopped spinach, 100 g (1 cup) of the mozzarella, 40 g (about 1/2 cup) of the hard cheese, 2 grated garlic cloves, the nutmeg, the finely grated zest of the lemon, 1/2 teaspoon of the fine salt, and the black pepper. Taste: the filling should be boldly seasoned. Only then stir in the beaten egg, which will bind it as it bakes.
5. Spread two thirds of the marinara over the bottom of a 9 x 13 inch (23 x 33 cm) baking dish.
6. Fill each shell with about 25 g (a heaping tablespoon) of filling, using a spoon or a zip-top bag with one corner snipped off, and nestle them side by side in the sauce, open side up, packed snugly.
7. Drizzle the remaining sauce over the shells, leaving some filling showing. Scatter the remaining 150 g (1 1/2 cups) of mozzarella and the remaining 20 g (1/4 cup) of hard cheese on top.
8. Cover the dish with foil and bake for 20 minutes: the trapped steam finishes cooking the pasta and heats the filling through. Remove the foil and bake for 15 more minutes, until the cheese is browned in spots and the sauce is bubbling at the edges.
9. Let rest out of the oven for 10 minutes: the filling firms up and the shells hold together when served. Scatter with fresh basil leaves and serve 6 shells per person.