Steps
1. Heat a pan over medium heat and add a drizzle of oil. Add the sliced onions and cook until caramelized, stirring occasionally.
2. Toast the bread slices until golden brown.
3. Spread mayonnaise or your preferred spread on one side of each bread slice.
4. Layer the caramelized onions and tomato slices on one bread slice. Sprinkle with salt and pepper.
5. Place the other bread slice on top to form a sandwich.
6. Cut the sandwich in half and serve.