Whisk together 2 tablespoons of the oil and the lemon juice in a bowl. Put the onion wedges in another bowl, drizzle over 1 tablespoon of this dressing and toss to coat. Set aside. Stir the garlic into the remaining dressing, then add the tomatoes and olives. Mix well and leave to marinate briefly.
Heat a cast-iron, ridged grill pan over a medium heat, then brush with the remaining oil. Add the onion wedges and cook for 3–4 minutes until nicely browned on the underside. Turn the onion wedges over with a spatula, then put the cheese slices into the pan. Increase the heat a little and cook the onion wedges for a further 3 minutes and the cheese for 1½ minutes on each side until tinged with brown and just beginning to melt at the edges. (Alternatively, cook the onions and haloumi on a sheet of foil under a medium grill.)
While the onion and haloumi are cooking, mix the spinach leaves and rocket together in a wide salad bowl. Leave in the bowl or serve onto individual salad plates. Arrange the onion wedges and haloumi on top.
Tip the tomato mixture onto the hot chargrill pan and stir for a few seconds until hot and steaming. Spoon over the haloumi and salad leaves and serve.
1½ portions vegetables
Haloumi cheese will toughen if allowed to cool, so it's important to serve as quickly as possible. If your pan isn't large enough to fit both the onion wedges and cheese slices together, cook the onion first, then pile at the edge of the pan while cooking the haloumi.