Melt 3 Tablespoons salted butter in a sauté pan over medium heat. Add the onion and sauté until golden brown and caramelized. Stir often and reduce the heat if the onions begin to brown too quickly. This will take about 20-30 minutes. Remove the onions and cover to keep warm. Wipe sauté pan clean.
Melt 1 Tablespoon butter in the same saucepan. Add bratwurst or knockwurst and sauté until golden brown on all sides and cooked through.
Serve bratwurst on buns topped with caramelized onions and Curry Ketchup.