Prepare the marinade by pressing garlic cloves through a garlic press and mixing with all other marinade ingredients. Place the pork belly and marinade in a freezer bag, seal, and refrigerate for 24 hours, turning the bag occasionally.
Preheat the oven to 180°C (356°F) with top and bottom heat. Line a baking pan with parchment paper. Arrange potato slices and onion wedges on the parchment paper. Place the pork belly skin-side down on the oven rack above the pan.
Roast for 1 hour. Turn the belly skin-side up and roast for another 1 hour, brushing with marinade several times during cooking, until the internal temperature reaches 63°C (145°F).