Crack eggs in a bowl, add milk/water and pinch of salt, whisk well and set it aside.
Heat butter in a nonstick pan with medium low heat. Meanwhile, toast the bread in a toaster.
When the pan is hot and the butter starts melting, pour in the egg mixture and stir the mixture several times until it starts to set then push the edge to the center area with a spatula.
When it's almost set, Fold the edge to the center to form a square that's similar to the size of your bread and top with a slice of cheese if using. Remove it from the pan and place it on a plate.
Assemble the sandwich
Spread nut butter on both slices of bread, place the omelet in the middle, put the sandwich together and enjoy!