Step 1: Warm the olive oil in a large skillet. Add the crumbled sausage and cook it for about 5 minutes, or until it is no longer pink and it is cooked through. Use a wooden spoon to break up the sausage into smaller pieces while it cooks. Remove from heat and transfer to a plate lined with a paper towel to cool and drain.
Step 2: Place the tomatoes, butter, onion halves, basil, and salt in a large saucepan and allow everything to simmer on very low heat for 45 minutes, stirring occasionally. After simmering is complete, discard the onion halves.
Step 3: Preheat oven to 350ºF. Boil the shells in salted water according to package directions. Drain and allow to cool until you can easily handle them.
Step 4: In a large bowl combine the ricotta, Parmesan cheese, and pepper. Mix it with a fork until well combined but still fluffy. Stir in the cooked sausage.
Step 5: Pour a thin layer of tomato sauce onto the bottom of a 9×13 baking dish. Then, scoop a heaping teaspoonful of the cheese mixture into each shell and line them up in the baking dish. Ladle a second layer of marinara sauce on top of the shells and sprinkle with Parmesan cheese.
Step 6: Bake the shells for 35 minutes, or until heated through. Serve warm.