Dry skin overnight: Place pork belly on a plate. Pat skin dry with paper towels. Leave uncovered in fridge overnight to dry out the skin.
Preheat oven to 140°C/285°F (120°C fan).
Season flesh: Drizzle flesh (only) with 1 tsp oil. Sprinkle over ½ tsp salt, the pepper and fennel powder. Rub all over the flesh, including on the sides.
Wrap in foil: Place 2 pieces of foil on a work surface. Put the belly in middle of foil, skin side up. Fold the sides in to enclose the belly, forming an open box.
Season skin: Pat skin dry with paper towels. Rub with ½ tsp oil then sprinkle remaining ½ tsp salt evenly all over the skin surface.
Slow roast 2 ½ hours: Place in the oven and roast for 2 ½ hours. After 1 ½ hours, remove and tighten the foil.
Increase heat: Remove pork from oven, then increase heat to 240°C/465°F.
Level pork: Remove pork from foil and place on a rack set over a tray. Use scrunched up balls of foil stuffed under pork to prop up so the skin surface is as level as possible.
30 minutes on high heat: Place the pork back in the oven for 30 minutes, rotating after 15 minutes as needed, until the crackling is deep golden, puffy and crispy all over.
Rest 10 minutes: Remove pork from oven. Rest for 10 minutes before slicing.
Serving: Slice into 2cm / ⅘"-thick slices using a serrated knife.
