Optionally heat the roll (e.g., in the oven). In the meantime, prepare the other ingredients. Heat the sausage in a pan, turning it over occasionally.

Wash and dry the lettuce. Cut slices from the washed and dried tomato. Cut the slices in half. Peel the onion and chop finely.

Cut the roll from the top along the middle (do not cut all the way through, so a "pocket" is formed). Inside, put lettuce, mustard, tomato, sausage, onion, and ketchup.