Dice the onion into small cubes. Melt 8 g butter in a saucepan over medium heat, then add the onion and fry for about 3 minutes.
Add 1 teaspoon sugar and let the onion caramelize for 1-2 minutes, stirring occasionally.
Deglaze the pan with 1 ½ tablespoon vinegar, scraping up any browned bits. Add 5 tablespoon ketchup, 2 tablespoon mustard, 1 teaspoon curry, and 1 teaspoon salt. Simmer on low heat for 60 seconds until the sauce is combined.
While the sauce cooks, heat oil or butter in a separate frying pan over medium-high heat. Fry the 4 sausages for 8-10 minutes, turning occasionally, until cooked through.
Meanwhile, bake the 4 rolls according to the package instructions until warm and crispy.
Cut each roll open lengthwise and place a sausage in each one. Spoon the sauce over the sausages and serve.