Heat the oil in a saucepan over medium heat. Cook the onion for 5 minutes until soft.
Add the tomato sauce, ketchup, curry powder, vinegar, brown sugar, Worcestershire sauce, cayenne, and salt. Simmer for 12 to 15 minutes until glossy and thick.
Cook the Sausage:
While the sauce simmers, brown the sausages in a skillet over medium heat until cooked through and deeply golden.
Rest the sausages for 3 minutes, then slice them into bite-sized coins.
Serve:
Spoon the curry sauce over the sliced sausage and dust generously with curry powder.
Serve immediately with hot crisp fries.
Notes
Use a mild curry powder for a classic sweet-savory profile, or add cayenne for more heat.
Currywurst is best assembled just before serving so the sausage and fries stay crisp.