Steps
1. Heat a skillet over medium heat.
2. Spread butter on one side of each bread slice.
3. Place the onion slices on one buttered side of a bread slice.
4. Place the tomato slices on top of the onions.
5. Cover with the other bread slice, buttered side facing up.
6. Place the sandwich in the skillet and cook for 2-3 minutes on each side, or until golden brown.
7. Remove from heat and let it cool slightly before serving.