Slice the halloumi into 8 thin slices.
Heat the oil in a frying pan or grill pan. When it’s hot, add the halloumi slices and cook each side for a few minutes until golden.
Meanwhile, stir the sriracha into the mayo.
To assemble, spread the bread with the sriracha sauce, add lettuce, tomato and onion and top with the grilled halloumi.
Finish with the top slice of bread. Serve and enjoy.