In a large non-stick skillet, toast the fennel seeds over medium heat, shaking the skillet constantly, until fragrant, about 60-90 seconds. Transfer to a large mixing bowl and set the skillet to the side.

Using a food processor, pulse 1 hoagie roll into fine crumbs (about 1/3 cup / 30g) and add to the bowl with the fennel seeds. Note: this uses one of your rolls, so you will have 4 rolls left for 5 servings — if you need all 5 subs, buy 6 rolls or use 1/3 cup (30g) store-bought plain breadcrumbs instead.

Chop up the onion (leave half of the onion in slices) and the garlic.

In the skillet from before, heat 1 tablespoon olive oil over medium heat. Add the garlic and one half of the chopped onion and cook, stirring, until softened, 3 to 4 minutes; add to the bowl.

Using the food processor, finely chop the basil, then add the pancetta and pulse until finely chopped. Transfer the basil-pancetta mixture to the bowl with the fennel seeds and breadcrumbs. Add the ground turkey, egg, oregano, pepper, and salt. Mix gently with your hands until just combined — stop as soon as everything holds together to avoid tough meatballs.

In the same skillet, heat 1 tablespoon olive oil over medium heat. Add the remaining sliced onions and cook, stirring occasionally, until browned and softened, about 7 minutes.

Shape the meat mixture into 16 meatballs and add to the skillet. Add a little more olive oil to make sure nothing sticks.

Stir in the tomato sauce, season with salt and pepper, and bring to a simmer. Cover with a lid and reduce heat to medium-low. Cook for 10 to 12 minutes, or until the meatballs reach an internal temperature of 165°F (74°C) when checked with an instant-read thermometer.

Preheat the broiler to high (500°F / 260°C). Position the oven rack 6 inches (15cm) below the heating element. Partially split the hoagie rolls; arrange cut side up on a baking sheet and drizzle with olive oil. Broil until toasted. You may need to use oven-safe dishes to keep them propped up.

Take the hoagie rolls out of the oven. Take one slice of mozzarella cheese, tear it in half, and line the bottom of the hoagie roll with cheese.

Top each roll with meatballs. Spoon some of the onion and tomato sauce over the meatballs. Take another slice of cheese, tear in half and add on top. Finish with shaved Parmesan.

Broil until the cheese is melted and just starting to brown, about 1-2 minutes. Watch the entire time — a broiler can take a sub from perfect to burnt in seconds. Pull them out as soon as you see golden-brown spots on the cheese.