Put the ribs on a rimmed baking sheet and cover tightly with foil; bake at 300°F for 2 1/2-3 hours, until tender. (Ribs can be cooked up to this point and then wrapped in foil and refrigerated for up to 3 days.) Meanwhile, stir together the hoisin sauce, vinegar, honey, garlic, ginger and hot pepper flakes.
Remove the foil, turn the oven up to 425°F, brush the ribs with sauce and roast for another 10 minutes, until dark and sticky. (Alternatively, you could finish them on the grill over high heat for a few minutes.) Sprinkle with sesame seeds and green onions.