In a blender, blend cilantro and serrano with 1/4 cup orange juice until mixture forms a smooth paste. Add remaining 3/4 cup orange juice, zest, tequila, broth and avocados and blend until smooth. Pour mixture through a coarse sieve into a large bowl. Season with salt and pepper.
Chill soup, covered, 2 hours, or until cold.
Serve topped with watermelon.
Recipe courtesy of Rosa Mexicana, NYC