In a small bowl or container, combine the olive oil, balsamic vinegar, salt, and pepper, if using.
In a large shallow bowl or serving platter, distribute the ingredients evenly and attractively. Spread a layer of half of the watermelon cubes or balls, then sprinkle over half of the onions, then feta. Tear half of the mint leaves into large pieces and sprinkle over the feta. Repeat with another layer of watermelon, onions, feta, and mint.
Shake or stir the dressing to re-combine, and pour it evenly over the salad. Serve chilled.
Notes
Change out the mint for another herb — try chervil, basil, or thyme.
Add 1 to 2 cups small diced seeded cucumber.
Replace the feta with crumbled goat cheese, diced fresh mozzarella, or ricotta salata.