Add ¼ thinly sliced medium red onion, 1 thinly sliced medium Persian cucumber, 2 pounds cubed seedless watermelon, ⅓ cup thinly sliced fresh mint, and 6 ounces cubed feta cheese to a large bowl.
Place 2 tablespoons rice vinegar, 1 tablespoon extra-virgin olive oil, ½ teaspoon kosher salt, and ¼ teaspoon black pepper in a small bowl and whisk to combine.
Drizzle the dressing over the salad and add 1 tablespoon lemon zest and 3 tablespoons lemon juice. Toss until combined. Garnish with 2 tablespoons coarsely chopped pistachios, more thinly sliced mint, and flaky salt.
