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Prepare to the end of step 4 a few hours ahead. Leftover salsa will keep in the fridge for a couple of days.
Cut the halloumi into 8 slices, put them in a shallow dish and drizzle with half the peri peri sauce, turning to coat. Set aside.
Mix the rest of the peri peri sauce into the mayonnaise with a squeeze of the lime juice, and set aside.
Cut the onion into 12 wedges through the root, to hold each wedge together. Heat a griddle pan on a high heat. Brush the onion with 2 teaspoons of the oil, add to the pan and cook for 2-3 minutes on each side until slightly caramelised, then remove to a plate.
Stand each corn cob upright on a chopping board and slice off the kernels. Heat the remaining 1 teaspoon oil in a large nonstick frying pan and toast the corn kernels over a high heat for 3-4 minutes until starting to char, then tip into a bowl. Stir in the spring onion, tomatoes, chilli, a little of the lime juice and some seasoning.
Toast the split buns on the hot griddle pan. Toss the avocado slices with a little of the lime juice. Spoon a dollop of the spicy mayo onto the base of each bun, then layer the lettuce, avocado and onions on top.
Add the halloumi to the hot nonstick frying pan and cook over a medium heat for 11⁄2-2 minutes each side until golden. Add to the buns, spoon some of the salsa on top and add an extra drizzle of peri peri sauce, if you like, before topping with the burger bun lid.