Preheat oven to 375 degrees F. In large frying pan, brown beef with onion, salt, and pepper. Stir in mushrooms and sour cream.
Separate crescent roll dough into 2 large rectangles. Place rectangles in ungreased 9x13 pan. Press over bottom and 1/2" up sides to form a crust. Spoon meat mixture into crust. Top with cheese slices.