Lay the pork belly flat on a work surface and use a small knife to create shallow crosshatching on the meaty side of the pork (the nonfat side).

In a small bowl, mix together the olive oil, garlic, rosemary leaves, fennel seeds, and orange zest. Season the pork belly generously with salt and pepper. Spread the oil and herb mixture all over the inside of the pork belly, massaging it into the crosshatches.

Starting from a short side, very tightly roll the pork belly into a log shape. Use butcher’s twine to tie the pork; space the ties 1–2 inches apart. Season the outside of the pork generously with salt. You can cook it at this point but it’s ideal to allow the pork to sit overnight uncovered in the fridge (place it on a Sheet Pan).

Remove the pork belly from the fridge 60–90 minutes in advance of cooking to allow it to come to room temperature. Preheat the oven to 325F.

Place the porchetta on a Roasting Pan Rack set into a Carbon Steel Roasting Pan and roast until the fat has softened and the internal temperature of the pork registers about 160F, about 3 hours.

Remove the Pan from the oven and carefully remove Rack with pork. Scatter the potatoes, spring garlic, fresh fennel, and rosemary sprigs into the Roasting Pan and season lightly with salt and pepper. Nestle the pork back into the Pan without the Rack. Return the Pan to the oven and roast until the vegetables are cooked through, 45–60 minutes.

Increase oven temperature to 400F and cook for an additional 15 minutes until the pork skin is crisp and the vegetables are golden.

Remove the porchetta from the oven and let rest for about 15 minutes before removing the twine and cutting. Serve slices of porchetta with the roasted vegetables.