In a small saucepan, combine the cornstarch and water. Bring to a boil; cook and stir for 1-2 minutes or until thickened.
In a blender or food processor, combine the lemon juice, ketchup, sugar, onion, Worcestershire sauce, salt, mustard, onion powder and garlic powder; cover and process until smooth. While processing, gradually add oil in a steady stream. Process until thickened. Transfer to a bowl; stir in cornstarch mixture. Cover and store in the refrigerator.