Mellow the Onion: Place the thinly sliced red onion into a small bowl of ice water and let sit for 10 minutes. Drain thoroughly and pat dry with paper towels.
Prep the Fruit and Base: In a large mixing bowl, gently combine the chilled diced watermelon, diced muskmelon, sliced kalamata olives, and drained red onion.
Whisk the Dressing: In a small cup or measuring glass, whisk together the extra-virgin olive oil, fresh lime juice, honey, salt, and black pepper until smooth.
Toss and Finish: Drizzle the dressing over the melon mixture. Add the chopped fresh mint and top with the crumbled feta cheese. Toss very gently to combine without breaking up the feta, and serve immediately while cold.