Make the marinade by mixing together the sugar, salt, soy sauce and cornflour slurry. Mix in the spare ribs, let marinate for 20 minutes.
Heat a large frying pan with 1 tablespoon oil over high heat. Brown the ribs on both sides, 3-5 minutes. Set aside.
In the same frying pan, heat 1 tablespoon oil over high heat. Stir-fry the onion with 1 teaspoon salt until soft, about 3-4 minutes. Add chopped tomatoes and ribs and mix well. Stir in the pasta sauce, water and sugar, bring to the boil. Turn the heat down to medium-low and simmer for 30-40 minutes. Adjust seasoning accordingly.