Rub the ribs all over with the five-spice powder; season generously with salt and pepper. Arrange the ribs in a single layer in a roasting pan and slow-roast for 2 1/2 hours.
To prepare the teriyaki glaze: Meanwhile, in a pot, combine the soy sauce, grapefruit juice, hoisin sauce, ketchup, rice wine vinegar, brown sugar, chile, garlic, and ginger over medium heat. Bring to a slow simmer and cook, stirring, until thickened, about 20 minutes.
In the last 30 minutes of cooking, baste the ribs with the teriyaki sauce. When they are done, the pork will pull away from the bone and you will see about 1/2-inch of bone showing. Just before you're ready to eat, baste the ribs with the teriyaki sauce again and stick them under the broiler for 5 to 8 minutes to make the spareribs a nice crusty brown. (Keep a close eye on these guys - ribs go from perfectly crisp to perfectly burnt seconds.) Separate the ribs with a cleaver or sharp knife, pile them on a platter, and pour on the remaining sauce. Sprinkle with sesame seeds, chopped cilantro, and green onion before serving.