Steps
1. Heat a skillet over medium heat.
2. Spread butter on one side of each bread slice.
3. Place the bread slices, butter side down, on the skillet.
4. Top one slice with cheese, ketchup, and sliced onions.
5. Place the other bread slice, butter side up, on top.
6. Cook until the bottom slice is golden brown, then flip and cook until the cheese is melted.
7. Remove from heat and let it cool for a minute.
8. Cut the sandwich diagonally into two triangles.
9. Serve hot and enjoy!