Heat the grill. Line a grill pan with foil and place the halloumi slices on it and grill for 3-4 mins, checking occasionally until golden.
Divide the guacamole between the 2 slices rye bread, spreading it evenly. Arrange 4 slices of tomato on each sandwich, and top with the Cajun grilled halloumi slices. Finish with a squeeze of lime and some ground black pepper.