Mix 1 tbsp minced garlic, 1 tbsp butter, and 1 tbsp sugar together in a bowl.

Microwave the mixture for 30 seconds to slightly melt the butter for easier mixing.

Chop the onion finely and soak it in water to remove its sharpness.

Mix the tuna, chopped onion, and 1 tbsp mayonnaise together. Add a pinch of black pepper to reduce the fishy taste.

Spread the garlic butter mixture on one side of each slice of bread.

Toast the bread slices in a pan over low heat until golden brown on both sides.

Sprinkle parsley flakes on the toasted bread for added aroma and presentation.

Spread the tuna mixture on one slice of bread and cover it with another slice.