Combine tomato sauce, Kikkoman Sweet Soy Sauce, garlic, ginger, oil and chilli flakes in a small bowl. Place ribs in a non-metallic flat dish and pour over the Sweet Soy mixture. Turn the ribs to coat evenly.
Cover with plastic wrap and place in fridge, turning occasionally for 4 hours or overnight for a deeper flavour
Preheat oven to 210°C. Pour enough water over the base of a baking dish to cover (to help prevent excess marinade from smoking). Brush a wire rack with the oil and place in baking dish over the water
Remove ribs from marinade and pat dry with paper towels to prevent burning. Arrange in a single layer on the wire rack. Reserve marinade
Bake ribs in preheated oven, turning and basting occasionally with the marinade
Cook for 40-50 minutes or until ribs are golden brown and sticky. Remove ribs from oven. Place on a heated serving platter and top with watercress slices. Serve immediately