In a saucepan, combine cornstarch and orange juice until smooth. Stir in the grapefruit juice, vinegar, marmalade, mustard, orange zest, garlic powder and onion powder. Bring to a boil; cook and stir for 2 minutes or until thickened.
Remove from the heat; refrigerate until chilled. Divide lettuce and fruit among four salad plates; drizzle with dressing.