Warm the oil in the saucepan over medium heat and sauté the onions until they are soft. Add the curry powder and stir until the powder absorbs the oil and the heat releases the fragrance.
Add the tomato sauce and sugar and stir. Continue cooking at a simmer until the sauce is heated through. Spoon over the sliced bratwursts, sprinkle on some more curry powder if you like, and serve hot.