In a large salad bowl, combine nectarines, onion, vinegar and sugar.
Season with salt and pepper and let sit for at least 10 minutes to marinate.
With a slotted spoon, strain the nectarines and onion and set aside, keeping the liquid for the dressing.
Pour the liquid into a measuring cup.
In the salad bowl, add the mixed greens, chopped mint, olive oil and add enough of the reserved liquid to coat the greens.
Cheese can be added to the salad and tossed or the salad can be placed on individual plates and topped with the cheese.