Heat oven to 375°F. Spray 11x7-inch (2-quart) glass baking dish with cooking spray. In 10-inch nonstick skillet, heat oil over medium-high heat. Add beef strips; cook 5 to 7 minutes, stirring frequently, until browned. Remove beef from skillet; place on plate.
In same skillet, cook carrots, celery and onion 10 minutes, stirring occasionally, until crisp-tender.
Return beef to skillet and stir in gravy, mustard and mushrooms. Cook about 2 minutes or until hot and bubbly. Pour into baking dish.
Separate dough into 8 triangles. Arrange triangles over filling, overlapping about 1/4 inch to create a braided appearance. Spray triangles with cooking spray; sprinkle with sesame seed.
Bake 22 to 24 minutes or until crust is deep golden brown.