Put the ribs in a large ziploc bag. In a bowl, whisk together the sherry, soy sauce, hoisin, brown sugar, garlic, Sriracha, Chinese 5-spice, onion powder, ginger, honey, sesame oil, and chicken stock, and pour into the bag with the ribs. Marinate overnight in the fridge.

Heat oven to 275F.

Remove the ribs from the marinade (reserve the marinade) and place on a rimmed bake sheet. Add 1/2 c chicken stock to the pan. Cover pan with foil and seal tightly. Bake for 1-1/2 hr.

While ribs are cooking, put the reserved marinade in a small saucepan and simmer until thickened to ~1/2c.

For the Chinese Honey Mustard: mix the mustard and honey in a small bowl. Serve on the side with the ribs.

Remove the ribs from the oven, discard the foil, and pour off the juices in the pan. Heat up the oven broiler. Baste the ribs generously on both sides with thickened marinade. Broil until glazed and charred, turning once (~2 minutes per side). Top with toasted sesame seeds and serve with the Chinese Honey Mustard.