Prick the skin and fat of pork belly with a sharp knife or ice pick, but don’t go through to the meat. Be sure to cover the entire surface of the skin with pricks.
Mix the salt, pepper, and paprika and rub the mixture onto the meat side of the pork belly. Avoid getting any on the skin side. Place into a container and rest overnight refrigerated.
Preheat oven to 350F.
Place the pork belly on top of a large sheet of aluminum foil skin side up and raise all the sides of the foil on the side of the pork to create a box. Brush the vinegar on the skin.
Layer the rock salt over the skin of the pork to completely cover without exposing any part of the belly.
Bake in the oven for 1 hour. Remove the pork out from the salt crust and place it on a sheet pan with a rack.
Raise the temperature of the oven to 450F. Bake the pork for another 20-25 minutes until the skin has been completely roasted. Remove from oven and let rest for 15 minutes before cutting.
Toss the apples with salt and sugar and place them onto a baking pan. Pour water over the apples and cover with aluminum foil.
Bake at 300F for 30 minutes, then transfer the apples to a blender and blend in a blender until smooth. Stir in the mustard and serve alongside the pork belly.