Combine onion and juice in a large zip-top plastic bag; seal and marinate in refrigerator 2 hours, turning bag occasionally. Remove onion from bag, reserving marinade.
Place potatoes in a saucepan, and cover with water; bring to a boil. Reduce heat, and simmer 25 minutes or until tender; drain. Cool. Peel and cut potatoes in half.
Place reserved marinade, cheeses, hot sauce, and milk in a blender; process until smooth.
Arrange 2 lettuce leaves on each of 6 salad plates. Top each serving with 2/3 cup onion, 3/4 cup potatoes, 2 tablespoons cheese sauce, and 2 egg wedges.