Slice Spam into 1/2-inch (1.2 cm) thick pieces. Heat 1 teaspoon of oil in a frying pan over medium heat, and cook the Spam until lightly browned on both sides.
In a bowl, lightly beat the eggs with salt and mirin. Heat 1 teaspoon of oil in a tamagoyaki pan (a rectangular Japanese rolled omelet pan), pour in half of the egg mixture, and cook into a square sheet, flipping to cook the other side as well.
Place a sheet of nori on a piece of plastic wrap with one corner pointing toward you. Put 1/2 cup of rice in the center and flatten it slightly. Place 1 piece of Spam, 1 sheet of cooked egg, and 1 lettuce leaf on top. Cover with another 1/2 cup of rice, and flatten again gently without squashing.
Bring all four corners of the nori to the center, then wrap the whole package tightly with plastic wrap. With the plastic still on, cut it in half.
Repeat with the remaining ingredients.