Tuna Sandwich

Add all the egg mixture ingredients to a bowl and mix well

Thinly slice the onion, sprinkle with plenty of salt (not included in the recipe), and use a cloth to rinse thoroughly to remove any spiciness. Add the tuna and mayonnaise and mix in a bowl

Heat 2 tablespoons of olive oil in a frying pan and pour in the egg mixture from [1]. Cook over high heat while shaking the frying pan, then cover with a lid, flip, cook for about 30 seconds, then turn off the heat and remove to a cutting board

Cut off the rounded corners of the omelet with a knife and divide it into four equal parts

Mix mustard paste and butter to make mustard butter, spread a generous amount on a slice of bread, and top with a slice of lettuce. Place a generous amount of (2) on top, then add the omelet, lettuce, and finally the bread to cover

Apply a little pressure with your hands to set it, then cut off the four sides with a knife and cut it into thirds

Turn it on its side and place it on a plate. Garnish with salad greens (not included in the recipe) to finish