Preheat the oven to 150°C/300°F, on convection mode.
In a bowl, combine the rosemary, thyme, garlic, crushed red pepper flakes, lemon zest, salt, and pepper, mixing well with a spoon.
Place the pork belly on a work surface with the skin side facing up. Using a sharp knife, score the skin vertically, leaving about 1cm (0.4 inches) of space between each cut.
Drizzle olive oil on both sides of the pork belly and evenly spread the herb mixture over it.
Roll up the pork belly and tie it securely with kitchen twine.
Fill a baking tray with water, place a rack on top, and set the pork belly on the rack.
Cover the tray with parchment paper and aluminum foil, then roast for 2.5 to 3 hours.
Switch the oven to grill mode, uncover the tray, sprinkle a bit of coarse salt on top, and roast again until the skin is golden and crispy.
Remove the porchetta from the oven, slice into pieces, and serve with apple sauce.