Place ribs in a large stockpot; fill pot with water until ribs are covered. Add onion, a couple of generous pinches of salt, and a few grinds of pepper. Bring ribs to a boil, then reduce heat to low and simmer 2 hours, or until ribs are very tender.
Preheat oven to 300 degrees F. Whisk together soy sauce, brown sugar, garlic and cornstarch.
Using tongs, carefully remove ribs from water (discard cooking liquid). Cut into individual portions (about 4 ribs per person) and place ribs, meaty side up, on a foil-lined baking sheet.
Brush with 1/3 of the sauce and bake 45 minutes, basting twice more with sauce (after 15 and 30 minutes).