The best Schnell-Imbisse (fast-food stalls) make their own currywurst sauce, essentially a curry powder–flavored ketchup. Heat 2 tbsp. canola oil in a saucepan over medium heat. Add 1 finely chopped large yellow onion; cook until soft, 8–10 minutes. Add 2 tbsp. curry powder and 1 tbsp. hot paprika; cook for 1 minute more. Using hands, crush 2 cups whole peeled canned tomatoes (with juice) into pan. Add 1⁄2 cup sugar, 1⁄4 cup red wine vinegar, and salt to taste; stir well. Increase heat to high; bring to a boil. Reduce heat to medium-low and simmer, stirring occasionally, until thickened, about 25 minutes. Purée sauce in a blender until smooth. Strain sauce through a sieve. Serve hot over sausage. Makes about 1 1⁄2 cups.