Make the balsamic reduction: Add balsamic vinegar and coconut sugar to a small saucepan over medium heat. Bring to a slight boil and reduce by half, about 6-8 minutes; set aside and let cool.
Mix remaining ingredients: In a large salad bowl gently toss watermelon, onion, feta, basil, olive oil and vinegar. Taste and season with pepper.
Serve: Drizzle balsamic reduction over salad just before serving. Serve immediately with a slotted spoon because the salad becomes juicy as it sits.
Notes
Vegan – If you need this salad to be vegan you can totally opt out of the feta cheese or use a dairy-free feta.
Basil – If you're not a fan of basil, mint would be a lovely substitution.
Balsamic reduction – If you don’t have time to prep the balsamic reduction, most grocery stores sell a bottled balsamic glaze or you can simply skip it. The salad will still taste delicious without the reduction. Cutting the balsamic reduction will also reduce the sugar content of the dish!