Heat half of the oil in a medium saucepan or skillet over medium heat. Sauté the onions until translucent and lightly golden (about 3 minutes). Add the tomato paste and cook for a few seconds.
Turn the heat down to medium-low, then add the curry powder and paprika and cook for a few seconds. Now add the tomato passata, honey, brown sugar, Worcestershire sauce, vinegar, salt, pepper, and cayenne (if using). Stir well, then simmer uncovered for 15-20 minutes while stirring occasionally.
Optional: Blend the sauce with an immersion blender (be careful, it may splatter).
Taste and adjust the sauce if needed (if it's too tomatoey, add more curry powder or vinegar; if it's too acidic, add more honey or sugar; if it's too thick, add a splash of water; if it's too thin, simmer with the lid off for a few more minutes).
While the sauce is simmering, heat the remaining oil in a large skillet over medium heat and fry the sausage until fully cooked and browned on all sides. Use a splash guard if you have one.
Slice the sausages (or leave them whole if you prefer), place them on a plate, and top with sauce. Then sprinkle with more curry powder. Serve right away with a crusty bun or French fries on the side.
Notes
Use either Bratwurst or a type of "Brühwurst" (look for Bockwurst or Knackwurst, sometimes also called Knockwurst, in the US and Canada; you could also use Weißwurst). If using Bratwurst, look for an authentic German-style bratwurst from a European deli or at least a regular Bratwurst without any extra flavors.
There are lots of different ways to make currywurst sauce, and your personal preferences might be different from ours. Use this recipe as a guideline and adjust it as needed.
Pre-measure all your sauce ingredients before starting. This will make it less stressful during cooking.
Be sure to finely chop the onion; you don't want large onion chunks in the sauce. If you want to use onion but don't want even small pieces in your sauce, you can blend it using an immersion blender after simmering (careful so it doesn't splatter).
Use a good-quality, mild curry powder, because some curry powders can be quite spicy when added in larger quantities.
Go easy on the cayenne pepper; you can always add more, but it’s hard to take the spice away.
We don’t usually cut slits into the Bratwurst before frying so that they stay as juicy as possible. If you’re worried about them bursting open, you can add diagonal slits before cooking.
You can also grill or air fry the sausages if you prefer.
You can make the sauce a day ahead and store it in the fridge. Just gently reheat the next day and add a splash of water if needed.
This nutritional information has been estimated by an online nutrition calculator. It should only be seen as a rough calculation and not a replacement for professional dietary advice.