Slice the halloumi into one-centimeter-thick slices and pat them thoroughly dry with paper towels. Moisture on the surface prevents the cheese from browning nicely.
Heat a grill pan or frying pan without oil until it is very hot and grill the halloumi for two minutes per side until golden-brown grill marks appear and the outside feels crispy. Remove it immediately, as overcooked halloumi becomes rubbery.
Cut the watermelon into chunks and arrange them with the arugula on a plate. Scatter the olives and coarsely chopped pistachios over the top.
Place the warm halloumi on the salad, drizzle with the olive oil and the juice of half a lime, and finish with torn mint and black pepper. The contrast between salty cheese and sweet melon keeps this light, while the halloumi and pistachios provide the protein and fat to keep you going until dinner.