Step 1Cut each rib strip between the bones or through the cartilage into individual riblets. Combine the onion, sugar, pepper and 2 tablespoons of the fish sauce in a bowl. Add the riblets, cover with plastic wrap and marinate in the refrigerator for at least 2 hours or overnight.
Step 2If necessary, adjust your broiler rack so that the ribs will cook as close to the flame as possible. Heat the broiler for 30 minutes to get it nice and hot.
Step 3While the broiler heats up, take the ribs from the refrigerator and let them sit at room temperature to take the chill off. Place them on a baking sheet and broil until they're tinged brown, about 4 to 6 minutes per side; a little charring is fine. (You'll hear a pleasant sizzle as this happens.) Alternatively, cook the ribs over high heat on a gas or charcoal grill, which imparts deeper flavor. The point here is to sear the riblets to obtain a roastiness and intensify the overall color.
Step 4Place the riblets in a saucepan with the Caramel Sauce, the remaining 2 tablespoons of fish sauce and enough water (about 2 1/2 cups) to cover most of the riblets. Bring to a boil and reduce the heat to simmer. Cover and let cook for 40 minutes; the ribs should simmer vigorously, sending steam out from under the lid.
Step 5Remove the lid and continue to simmer until the ribs are tender (you can easily pierce the meat with a fork or knife tip), about 20 to 30 minutes. If there's cartilage, you should be able to bite through it, with a slight crunch remaining. This latter phase of cooking allows the sauce to reduce and concentrate in flavor, and deepens the color to dark reddish brown. In the end, there should be a fair amount of sauce left.
Step 6Turn off the heat, tilt the saucepan so the liquid goes to one side and use a spoon or small ladle to skim the fat from the top. Adjust the flavors with extra fish sauce, if necessary. Garnish with the chopped green onion and serve with lots of steamed rice.