Heat oven to 375°F. In 12-inch skillet or Dutch oven, brown ground beef and onion over medium heat until beef is thoroughly cooked, stirring frequently. Drain.
Add potatoes, mixed vegetables and gravy; mix well. Bring to a boil. Reduce heat to medium; cover and cook 8 to 10 minutes or until vegetables are tender, stirring occasionally.
Spoon hot beef and vegetable mixture into ungreased 2 1/2 to 3-quart shallow casserole or 13x9-inch (3-quart) baking dish. Separate crescent dough into 8 triangles. Place triangles over hot mixture with points in center and wide ends around outer edge of casserole.
Bake at 375°F. for 14 to 17 minutes or until crescents are golden brown and mixture is bubbly around edge.