How to Make It
Process olives, walnuts, parsley, lemon zest, and 3 teaspoons of the lemon juice in a mini food processor until finely chopped, about 5 seconds. Transfer olive mixture to a small bowl, and whisk in 4 tablespoons of the olive oil until blended. Place watermelon on a baking sheet, and sprinkle evenly with 1/2 teaspoon of the salt.
Toss together arugula, and remaining 1 teaspoon lemon juice, 1 tablespoon olive oil, and 1/8 teaspoon salt in a medium bowl. Arrange arugula on a large platter. Nestle watermelon cubes in arugula mixture. Spoon olive mixture over watermelon; sprinkle feta and mint evenly over salad.