1Make ricotta cheese: In a saucepan, combine milk, heavy cream, and salt. Heat gently over medium-low heat, stirring frequently, to 190°F/88C. Remove from heat, stir in vinegar, and let sit for 15 minutes.
2Cook pasta shells in salted boiling water until almost al dente, about 6 minutes. Drain and toss with olive oil.
3Prepare the filling: Stack prosciutto slices, and mince into small pieces. Fry in olive oil in Dutch oven for 90 seconds. Add garlic and cook for 1 minute, then stir in spinach until wilted.
4Prepare the marinara sauce: In the Dutch oven, saute grated onion and garlic for 5 minutes. Stir in the Calabrian chiles and cook for 2-3 minutes.
5Add tomato paste and cook for 1 minute. Add crushed tomatoes, salt, sugar, water, and simmer for 5 minutes. Stir in basil and taste.
6Combine the ricotta, prosciutto-spinach mixture, heavy cream, mozzarella, parmesan, pepper, and egg in a medium bowl. Stir well to combine.
7Preheat the oven to 425°F/220C. Fill each pasta shell with the ricotta mixture and place in a Dutch oven with the marinara sauce.
8Sprinkle parmesan cheese over the stuffed shells. Cover the Dutch oven with a lid and bake for 15 minutes.
9Remove the lid and bake for another 15 minutes to brown the cheese.
10Serve the stuffed shells topped with extra parmesan and a drizzle of spicy olive oil.