There are two slightly different methods of cooking the currywurst sauce, depending on how much time you have.
The quick curry sauce
Finely chop the onion and apple. If you have time, grate the apple.
Pour a little olive oil in a saucepan and add the onion and apple. Cook, stirring frequently until the onion is glassy.
Add the other ingredients and stir well. If you like your curry sauce to be a little spicy, add 1/4 to 1/2 teaspoon chilli powder and an equal amount of paprika.
Add a little water, just to get the desired consistency, and cook over medium heat until heated through and well combined.
While the sauce is cooking, grill or barbeque the bratwurst. Cut into bite-sized pieces.
Serve the bratwurst with a good spoonful or two of sauce. Sprinkle a little curry powder over the sauce before serving.
The longer version
Complete steps 1-3.
Add a cup of chicken stock to the saucepan, bring to the boil and reduce to low heat. Simmer uncovered for about half an hour until most of the stock has evaporated and the sauce has the desired consistency.
Meanwhile, complete steps 5 and 6.
If you can't get bratwurst, use another plain sausage, preferably with a natural casing. Don't use hotdogs or other sausages that are designed to be boiled or warmed in water.Add as much or as little chilli powder as you like to make it as spicy as you prefer. Add the equivalent amount of (sweet) paprika to balance. Make sure you do a taste test as you may need to add more curry powder.Serve with hot chips or fries, or with a crusty (white) bread roll.