Preheat a sandwich press or toastie maker.
In a small frying pan over medium heat add the oil, sugar and onion. Cook over medium low, stirring often, until onions caramelise.
Meanwhile, butter the outsides of the bread. Spread the pesto on the insides of the bread, then layer on the turkey, tomato and mozzarella cheese. Add the caramelised onions then top with the other slice of bread.
Toast in your sandwich press or toastie maker until golden brown outside and cheese is melted.
If you don't have a sandwich press
You can just fry this like a regular grilled cheese if you prefer.
Don't like turkey?
You can leave it out for a veggie sammie or use chicken. Great way to use up leftover chicken.