Take one egg and one tablespoon of pesto, whisk until combined.
In a hot pan put a teaspoon of olive oil and swirl it around to coat the pan. Dump in the egg mixture. Tilt the pan so that it spreads out thin. Cook it for one minute and then gently fold in the sides to the middle, so that it forms a square shape (this makes it easier to fit on the bread).
Put the square omelet on a piece of bread; top with 1 ounce of mozzarella cheese. Top with another slice of bread. Repeat with remaining eggs and bread. Toast the sandwiches on both sides in the frying pan to melt the cheese. Serve with orange slices.