Toast the bread slices and set aside.

Melt 1 tbsp of butter in a pan over medium heat.

Fry the eggs in the melted butter to your desired doneness. Set aside.

Melt the remaining 1 tsp of butter in the same pan.

Add the chopped onion to the pan and sauté for about 1 minute until softened.

Add the tomato sauce, chili sauce, sherry vinegar, soy sauce, and sugar to the pan.

Bring the sauce to a boil, then reduce heat and simmer for a minute until slightly thickened.

Assemble the sandwich by spreading the sweet and sour onion sauce on one slice of toasted bread.

Place the fried egg on top of the sauce.

Cover with the other slice of toasted bread and serve immediately.