1.Heat half the oil on barbecue flat plate; cook onion, turning constantly, until browned lightly. Sprinkle onion with sugar and vinegar; cook, turning constantly, until onion is caramelised. Transfer onion to small bowl; cover to keep warm.
2.Heat remaining oil on flat plate; cook mushrooms, turning, until browned and tender.
3.Meanwhile, cook steaks on flat plate until cooked. Cover; stand while toasting bread, both sides, on flat plate.
4.Sandwich pâté, mustard, rocket, mushrooms, steak and onion between bread slices.
We used a simple port and chicken-liver pâté in our sandwich, but you can use whatever type of pâté you like best.