In a large nonstick skillet, toast the fennel seeds over medium heat. Transfer to a large bowl and reserve the skillet. Using a food processor, pulse 1 hoagie roll into crumbs; add 3/4 cup crumbs to the bowl.
Using the food processor, finely chop 1 cup onion slices and the garlic. In the reserved skillet, heat 1 tablespoon olive oil over medium heat. Add the garlic-onion mixture and cook, stirring, until softened, 3 to 4 minutes; add to the bowl, wiping out the skillet. Using the food processor, finely chop 1/4 cup basil; add the pancetta and pulse until finely chopped. Add to the bowl along with the turkey, egg, oregano and 1/2 teaspoon salt; mix together.
Shape the meat mixture into 16 meatballs. In the same skillet, heat 1 tablespoon olive oil over medium heat. Add the remaining onions and cook until browned, about 7 minutes; transfer to a bowl. Add the remaining 2 tablespoons olive oil and the meatballs and cook until browned, about 5 minutes. Stir in the tomato puree and reserved onions, season with salt and pepper and bring to a simmer. Cover with foil and cook until the meatballs are firm, 10 to 12 minutes.
Preheat the broiler. Partially split the remaining 4 hoagie rolls; arrange cut side up on a baking sheet and drizzle with olive oil. Broil until toasted. Top each roll with 4 meatballs. Chop the remaining 1/2 cup basil and stir into the tomato sauce; spoon over the meatballs. Sprinkle with the mozzarella and broil until the cheese is melted.