Make a salad from the onion, tomatoes, basil and sumac, and dress with salt, pepper, olive oil and vinegar. Set aside.
Heat a non-stick frying pan. Lightly dust the cheese with flour, shaking off the excess. Pour olive oil into the pan and add the slices of haloumi. Cook quickly until golden on both sides then remove to a platter.
Squeeze lemon juice over the cheese, then scatter the salad over and serve.