Beef Stroganoff Pockets

1 (4 ounce) can mushroom stems and pieces, drained
1 1/2 cups leftover beef gravy or 1 (18 ounce) jar prepared beef gravy
1/2 cup sour cream
8 thin slices roast beef
4 whole pitas

In a large skillet over medium heat, stir mushrooms and gravy until hot. Stir in sour cream until smooth. Add roast beef slices and cook until heated through. Top each slice of bread with two roast beef slices and some gravy and serve open-faced.