Heat the grill to medium-high. Put the burgers on a baking sheet and grill for 5-8 mins on each side until cooked through. For the last minute, top the burgers with the cheese slices and grill until melted. Lightly toast the burger buns. Divide the guacamole, burgers, chipotle or chilli mayo, salsa and nachos, broken up, between each bun. Finish with a sprinkling of torn coriander.