Peel and finely chop the red onion. Trim and dice the cucumber. Roughly chop the tomatoes into small pieces. Halve the chilli, flicking out the seeds and membrane for less heat, and finely slice it. Stir all 4 ingredients together in a large bowl.
Finely slice the coriander stalks (save the leaves for later) and slide into a small bowl. Grate in the lime zest and squeeze in the juice. Add a pinch of salt and pepper and 1 tbsp oil. Whisk together to make a dressing.
Pour 1 tbsp oil into a frying pan and bring to a medium-high heat. Slice the halloumi into rough 2cm cubes and carefully add to the hot pan. Fry for 5 mins, turning frequently, till the cheese is golden brown on all sides. Tip onto a plate lined with kitchen paper to drain.
Spoon the lime dressing into the bowl of chopped salad and stir well.
Separate the lettuce leaves and arrange on a couple of plates. Fill each lettuce scoop with the chopped salad. Top with the fried halloumi and finish with the reserved coriander leaves.