Preheat oven at 425°. In a large skillet, cook the beef, mushrooms and onion over medium heat until meat is no longer pink, 5-7 minutes; crumble meat; drain. Stir in salt and pepper; set aside.
On a lightly floured surface, roll crust into a 15x12-in. rectangle. Transfer the crust to a parchment-lined baking sheet.
In a small bowl, combine the sour cream, Italian seasoning and garlic powder; spread over crust to within 2 in. of edges. Spoon beef mixture over top. Fold up edges of crust over filling, leaving center uncovered.
Bake for 15-18 minutes or until crust is golden. Using the parchment, slide tart onto a wire rack. Sprinkle with blue cheese; let stand for 5 minutes before slicing.
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