In a small mixing bowl, combine the sugar, ketchup, lime juice, vinegar, soy sauce, and fish sauce. Stir to combine thoroughly and set aside.Heat the canola oil in a large (12-inch) stainless steel or cast-iron skillet over medium-high heat. Add the red onion and stir-fry until softened, but not quite beginning to brown, about five minutes, stirring often with a bamboo spatula or wooden spoon.Add the ground beef to the skillet. Break up the meat with a spatula it cooks, stir-frying until it has cooked through and most of the liquid has evaporated, about 5-7 minutes. Add the peppers to the skillet and stir-fry until al dente, about five minutes.Add the tomatoes and stir-fry until slightly softened but still whole, about three minutes.Pour the sauce mixture into the skillet and mix to coat the ingredients evenly. Let simmer until thickened slightly, about two minutes. Serve over steamed rice.