Watermelon Surprize

6 cups cubed seeded watermelon

½ red onion, cut into thin half-moon slices

⅓ cup apple cider vinegar

2 tablespoons chopped fresh mint, or more to taste

½ teaspoon ground black pepper

Toss watermelon, red onion, apple cider vinegar, mint, and black pepper together in a large mixing bowl. Refrigerate until chilled completely, about 1 hour.