Press the garlic clove, and put it in a small bowl. Add olive oil, salt, sugar, pepper, and lemon juice to the bowl and mix.
Taste, and add drops of vinegar if it's not acidic enough. Leave at room temperature while you prepare the other ingredients (the dressing can also be pre-made and left covered in the fridge for up to a few days)
Cut the onion in rounds and slice the halloumi. Heat a thick-bottomed frying pan on medium-high heat.
Brush the pan with olive oil. Place the onion rounds in the pan, taking care not to separate the individual layers from each other.
Carefully add the halloumi slices to the pan. Don't move them for at least 30 seconds.
After 30 seconds, check if the halloumi slices can be flipped without tearing apart. Cook until golden brown on both sides, then remove them from the pan and leave to cool.
After about a minute, check if the onion is ready to flip. When it's slightly charred on both sides, take the pan off the heat.
Cut the nectaries in 1/2 cm thick slices. Toss them with a couple of teaspoons of the dressing.
Wash and dry the basil leaves. Tear big leaves into smaller bits.
Arrange nectarines, halloumi, and basil leaves on a plate or small serving platter. Separate the onion rings from each other and place on top of the salad. Drizzle with the dressing.
Serve the salad at room temperature. This portion serves two as a side or an appetizer. If you were to add a few slices of garlic bread to this, it would also make a good lunch for two.