Steps
1. Heat a skillet over medium heat.
2. Spread butter on one side of each bread slice.
3. Place one slice of bread, buttered side down, on the skillet.
4. Spread ketchup and mayonnaise on the other side of the bread.
5. Add sliced onions on top of the condiments.
6. Place the cheese slices on top of the onions.
7. Cover with the second slice of bread, buttered side up.
8. Cook for 2-3 minutes, until the bottom bread slice is golden and the cheese starts to melt.
9. Flip the sandwich and cook for another 2-3 minutes, until the second side is golden and the cheese is completely melted.
10. Remove from heat and let it cool slightly before serving.