Don't be put off by the unusual combination of ingredients; the sweet-salty pairing of melon and feta cheese is wonderfully compelling. Any variety of melon (or an assortment) will work; just be sure to choose ripe, fragrant and juicy specimens for the best results.
Combine the melon, feta, scallions and basil in a large bowl and toss gently to combine. Refrigerate the mixture until serving time.
Place the oil, vinegar, salt and pepper in a small glass jar with a lid and shake vigorously to combine.
Just before serving, pour the vinaigrette over the melon mixture, tossing to combine. Serve immediately.