Cut the watermelon into 1-inch cubes and spread in a single layer on a rimmed sheet pan or a plate lined with paper towels; let drain for 8–10 minutes to remove excess juice.

In a small bowl whisk together the lime juice, lemon juice, olive oil, honey, salt, and black pepper until combined; stir in the chopped mint and taste, adjusting acidity or sweetness as needed.

Just before assembling, peel and cut the avocados into 3/4-inch chunks and crumble the feta cheese if not pre-crumbled.

Transfer the drained watermelon to a large serving bowl, add the avocado chunks and crumbled feta, sprinkle the sliced red onion if using, pour the dressing over the salad, and gently toss once or twice until evenly coated; garnish with torn mint leaves and serve immediately.

If using cooked quinoa, fold it in gently with the watermelon before adding avocado; if adding jalapeño, stir it into the dressing or scatter thin slices over the finished salad.