For the herb mixture
Place the rosemary, thyme, garlic, lemon zest, salt, and pepper in a bowl and mix well with a spoon.

For the meat
Place the pork belly on your work surface with the skin side up. Using a sharp knife, score the skin vertically, leaving a 1 cm gap between each cut.
Flip the meat over and score the flesh in a diamond pattern. Using a brush, spread the ketchup-mayo over the meat and sprinkle with the herb mixture.
Roll up the pork belly and tie it securely with kitchen twine. Sprinkle coarse salt over the outside of the roll and place it uncovered on the top shelf of the refrigerator for 8 hours.
Preheat the oven to 170°C (fan setting).
Fill a baking tray with water, place a rack over it, and set the pork roll on top. Brush the surface with sunflower oil, removing any excess salt at the same time.
Cover the tray with parchment paper and aluminum foil and roast for 2.5 hours.
Switch the oven to grill mode, uncover the tray, and roast again until the skin is golden and crispy.
Remove the porchetta from the oven and allow it to cool slightly.

For the potatoes
Heat a frying pan over medium-high heat, add the butter, 1 tbsp olive oil, rosemary, thyme, and garlic, and let the aromas release for 1 minute.
Lightly press each boiled potato with your hand or a fork to slightly flatten, then add them to the pan and cook until golden and crispy.

For serving
Slice the porchetta into portions and serve with ketchup-mayo, accompanied by the crispy potatoes.