super easy and yummy MUSHROOM TURNOVERS

I found this great mushroom turnover recipe in Martha Stewart's Everyday Food magazine. It is a fabulous vegetarian main dish, and extremely easy & quick to make.

• 2 tablespoons olive oil
• 1 large onion, chopped
• 4 packages (5 ounces each) mixed mushrooms, trimmed and thinly sliced
• Coarse salt and ground pepper
• 1 sheet frozen puff pastry (from a 17.3-ounce box), thawed OR 1 package refrigerated pie dough
• 1/2 cup reduced-fat sour cream, for serving

1. Preheat oven to 425 degrees. In a large nonstick skillet, heat oil over medium-high. Add onion; cook, stirring occasionally, until golden brown, 5 to 6 minutes. Add mushrooms; cook, stirring occasionally, until mushrooms are tender, 4 to 5 minutes. Season with salt and pepper; set aside.
2. On a lightly floured work surface, roll out dough (still folded in thirds) to a 20-by-10-inch rectangle. Using a pizza cutter or sharp knife, trim edges straight; cut dough into four 10-by-5-inch rectangles. Dividing evenly, mound mushroom mixture on one half of each rectangle, leaving a 1/2-inch border on three sides. Lightly brush border with water; fold other half of pastry over filling, and press edges firmly to seal. With a floured fork, crimp edges.
3. Using a wide metal spatula, transfer turnovers to a large baking sheet. With a paring knife, cut 3 to 4 small slits in the top of each turnover. Bake until golden and puffed, 20 to 25 minutes. Let cool 5 minutes; serve turnovers with sour cream.