1. Stir together first 5 ingredients and 1/4 cup water in a glass bowl. Cover and chill 2 hours. (Mixture can be made and chilled up to 2 days ahead.) Remove onions from marinade, discarding marinade.
2. Toss together greens, cheese, 1 cup red onions, and desired amount of dressing in a large bowl. Top each watermelon slice with 1 1/2 cups greens mixture. Sprinkle with pepitas. Serve immediately with remaining vinaigrette and onions.