To prepare the vinaigrette, combine water, juice, and cornstarch in a small saucepan, stirring with a whisk. Bring to a boil over medium-high heat; cook 1 minute, stirring frequently. Remove from heat. Stir in honey, vinegar, salt, and pepper. Place in a blender or food processor. Add cheese, and process until well blended. Cool to room temperature.
To prepare salad, combine lettuces and onion in a large bowl. Drizzle with vinaigrette, tossing gently to combine. Serve immediately.