For the curry ketchup sauce: In a medium pot on medium heat, add the ketchup, honey, curry powder, a pinch of salt and a few grinds of black pepper. Cook while stirring until the curry powder melts into the ketchup and the red color deepens, about 5 minutes.
For the wurst and onions. In a large pot on medium-low heat, add the wurst in 1 layer, then pour the stock over the top. Cover, bring just to a simmer and gently simmer until the wurst is cooked through, 15 to 20 minutes.
Preheat a grill to high heat. Remove the wurst from the pot, place on the grill, drizzle with a little olive oil and cook until charred on both sides and slightly bursting, 8 to 12 minutes total. Remove to a cutting board and cover with aluminum foil. Toss the onion slices with olive oil, add to the grill and cook until charred on only one side, about 5 minutes. Remove the onions to a cutting board and cover with aluminum foil for 10 minutes. Cut each wurst link into 1/2-inch discs and chop the onion slices.
Open the rolls by splitting them across the top, then scoop the wurst and onions into the rolls. Sprinkle with curry powder and drizzle with the curry ketchup sauce.