Heat oil in pan over medium-high heat. Add onion. Sauté until soft. Spoon into a small bowl and set aside.
Add butter and mushrooms to pan. Sauté until golden. Add onion to mushrooms. Sprinkle with flour, salt, pepper, and garlic powder. Mix well. Stir in sour cream until mixture is thickened. Cool.
Roll out pastry on a lightly floured surface to rectangle measuring 11 x 13 inches. Cut lengthwise down the center, forming 2 long rectangles. Divide mushroom and onion mixture between the 2 rectangles. Spread evenly, leaving 1/2 inch space down each side.
Roll each long side to meet in center, just touching each other. Dampen pastry along each edge and press together lightly. Wrap in plastic wrap and chill for 30 to 40 minutes in the refrigerator (or 15 minutes in the freezer).
Preheat oven to 375°F. Lightly grease a baking sheet.
Cut each chilled roll into 24 slices, each slice about 1/2 inch wide. Arrange on prepared baking sheet., cut side down. Lightly press down with a spatula or your fingers.
Bake in upper third of preheated oven for about 12 minutes until golden. Serve warm.