Heat a pan over medium heat and add a little butter.
Crack the eggs into the pan and cook to your preference (scrambled or fried). Season with salt and pepper.
Place the cheese slice on the eggs while still in the pan and let it melt slightly.
Lightly butter the bread and toast it until golden.
Spread mayonnaise or ketchup on one side of the bread (optional).
Place the cooked eggs with melted cheese onto the bread.
Add onion and tomato slices if using.
Top with the second slice of bread and gently press.
Optional: Toast the assembled sandwich in the pan for 1–2 minutes for extra crispiness.
Serve hot.