Pat the pork belly dry. If skin-on, lightly score the skin, taking care not to cut deeply into the meat. Season well all over with salt and black pepper.
2 pound pork belly, 1 teaspoon salt, ½ teaspoon black pepper
Add the sliced onions and apple wedges to the bottom of the slow cooker. Place the pork belly on top, skin/fat-side up.
2 apples (Granny Smith or another tart apple), 2 large onions
Whisk together the chicken stock, minced garlic, Dijon mustard, Worcestershire sauce, apple cider vinegar, thyme and sage. Pour the mixture around the pork belly, avoiding wetting the top as much as possible.
4 garlic cloves, 1 cup low-sodium chicken stock, 1 tablespoon Dijon mustard, 1 tablespoon Worcestershire sauce, 1 tablespoon apple cider vinegar, 1 teaspoon dried thyme, ½ teaspoon dried sage
Cover and cook on LOW for 7-8 hours, until the pork belly is very tender but still holds together well enough to slice.
Stir together the maple syrup, wholegrain mustard, Dijon mustard, apple cider vinegar and a pinch of salt.
2 tablespoons maple syrup, 1½ tablespoons wholegrain mustard, 1 teaspoon Dijon mustard, teaspoon apple cider vinegar, Pinch of salt
Carefully transfer the cooked pork belly to a foil-lined baking sheet. Pat the top dry, then brush generously with the maple mustard glaze. Place under a high broiler for 3–5 minutes, watching closely, until the glaze is bubbling, sticky and deeply browned.
Tent the pork loosely with foil and let it rest for 10 to 15 minutes.
Make the cornstarch slurry by mixing the cornstarch with a little cold water.
1 to 2 tablespoons cornstarch, 1 to 2 tablespoons cold water
Strain some of the slow cooker cooking liquid into a saucepan and skim off as much excess fat as possible. Bring to a simmer. Gradually stir the cornstarch slurry into the cooking liquid. Simmer until thickened to a gravy consistency, adding more slurry if required. Taste and season with salt and pepper as needed.
1 cup strained slow cooker cooking liquid, excess fat skimmed off
Cut the pork belly across the grain into thick slices. Serve with the gravy and the slow-cooked apples and onions.