Slice the Vienna sausages diagonally.
Cut the onion and bell peppers into bite-sized pieces.
Add cooking oil to a pan and stir-fry the sausages.
Add the onion and bell peppers and stir-fry together.
Add ketchup, gochujang (Korean chili paste), and sugar, then stir-fry everything evenly.
Taste and adjust the seasoning to your preference.