Heat oil in large nonstick skillet over medium heat and cook onions, stirring occasionally, until tender, about 6 minutes. Meanwhile, combine water and Knorr® Vegetable Bouillon Cube in medium bowl; set aside.
Stir flour into skillet and cook, stirring, until flour is toasted, about 1 minute. Gradually whisk in Bouillon mixture and bring to a boil. Boil until thickened, about 2 minutes. Stir in turkey and vegetables and cook, stirring occasionally, until heated through, about 3 minutes.
Transfer to 2-quart (2 L) baking dish. Top with hot mashed potatoes and sprinkle with cheese. Broil until golden, about 3 minutes.