Steps
1. Peel the potato and cut it into thin slices.
2. Heat oil in a pan and fry the potato slices until they turn golden and crispy.
3. Slice the cucumber and onion into thin rounds.
4. Toast the bread slices until they are crisp and golden.
5. Spread ketchup on one side of a bread slice and mayonnaise on the other.
6. Layer the fried potato slices, cucumber, and onion on one of the bread slices.
7. Top with the other bread slice and press gently to secure the sandwich.
8. Cut the sandwich in half and serve.