Mix the vinegar and Dijon mustard in a bowl. Stir the garlic, salt, and pepper into the mixture. Slowly stream the olive oil into the dressing while whisking vigorously. Place the dressing in the refrigerator until ready to use.
Combine the watermelon, feta cheese, and red onion in a large bowl; toss lightly to mix. Season with the coarsely ground black pepper. Pour about half the dressing over the salad; gently toss to coat. Refrigerate the salad at least 30 minutes. Drizzle the remaining dressing over the salad just before serving.