For the tomato sauce
1 28-oz. can Italian plum tomatoes
4 oz. (8 Tbs.) unsalted butter, cut into pieces
1 medium onion, peeled and halved
Pinch granulated sugar
Kosher salt
For the besciamella sauce
2-1/2 oz. (5 Tbs.) unsalted butter
1 3/8 oz. (5 Tbs.) unbleached all-purpose flour
5 cups whole milk
1 small bay leaf
1/4 tsp. freshly ground white pepper
1/8 tsp. freshly grated nutmeg
Kosher salt
For the filling
Kosher salt
Olive oil as needed
2 large eggs
4 cups homemade ricotta (drained for 30 minutes to 2 hours) or good quality purchased
8 oz. (2 cups) coarsely grated fresh mozzarella; more for topping (optional)
4 oz. prosciutto cotto, cut into 1/4-inch dice (3/4 cup)
1-1/4 oz. (1-1/4 cups) finely grated Parmigiano-Reggiano
3/4 oz. (3/4 cup) finely grated pecorino romano
1/3 cup finely chopped fresh flat-leaf parsley
Freshly ground black pepper
1 lb. dried jumbo pasta shells