Steps
1. Heat a non-stick pan over medium heat.
2. Spread ketchup on one slice of bread.
3. Layer the mashed potatoes on top of the ketchup.
4. Sprinkle shredded cheese over the potatoes.
5. Add the sliced onions on top of the cheese.
6. Place the other bread slice on top.
7. Carefully transfer the sandwich to the heated pan.
8. Cook for 2-3 minutes on each side, until the bread is golden brown and the cheese is melted.
9. Remove from the pan and let it cool slightly before serving.