1.Heat 1 tablespoon of the oil in a large frying pan over medium heat; cook onion, stirring occasionally, 8 minutes or until softened. Remove from pan; cover to keep warm.
2.Meanwhile, make lemon mint mayonnaise. Combine mayonnaise, mint and lemon juice in a small bowl.
3.Heat a chargrill pan over high heat. Trim ends from bread; cut remaining bread into quarters then halve pieces horizontally. Spray cut sides of bread with cooking oil. Place bread, cut-side down, in pan for 30 seconds until charred and toasted.
4.Combine halloumi , harissa and remaining oil in a medium bowl. Heat frying pan over high heat; cook halloumi 1 minute each side or until golden.
5.Spread bread with mayonnaise. Sandwich with tomato, mesclun, halloumi and onion.
Halloumi is a firm, cream coloured sheep-milk cheese; it tastes like a minty, salty feta in flavour, and can be grilled or fried, briefly, without breaking down. It should be eaten while still warm as it becomes tough and rubbery on cooling.Note