1Make ricotta cheese by heating milk, cream, and salt to 190°F. Remove from heat and gently stir in vinegar; let sit for 15 minutes.
2Boil salted water; add jumbo pasta shells, cook for 6 minutes until al dente. Drain, and toss gently with olive oil.
3Prepare the prosciutto and spinach filling by mincing prosciutto, sauteing in olive oil, adding minced garlic, and wilted spinach. Chop finely and squeeze out excess water.
4Prepare the marinara sauce by sauteing grated onion and minced garlic in olive oil until softened. Add Calabrian chiles, tomato paste, and simmer for 1 minute. Add crushed tomatoes, salt, sugar, and water; simmer for 5 minutes. Stir in basil and taste, adjust seasonings.
5Combine ricotta, prosciutto-spinach mixture, heavy cream, mozzarella, parmesan, pepper, and egg. Stir until well combined.
6Preheat oven to 425°F. Fill each shell with 2-3 tablespoons of the ricotta filling. Arrange shells in the marinara sauce, ensuring they are halfway submerged.
7Sprinkle parmesan cheese over the shells. Cover with a lid and bake for 15 minutes. Remove the lid and bake uncovered for another 15 minutes.
8Serve by scooping 3-4 shells onto a plate, top with parmesan and drizzle with spicy finishing olive oil.