Preheat oven to 400°F (200°C).

Heat olive oil in a skillet over medium heat. Sauté diced onion until translucent. Add minced garlic and sliced mushrooms; cook until mushrooms are browned.

Stir in chopped sage, salt, and pepper until fragrant.

Roll out puff pastry on a floured surface; cut into four-inch squares.

Place a spoonful of the mushroom filling in the center of each square. Fold diagonally to form triangles and seal edges with a fork.

Arrange turnovers on a parchment-lined baking sheet. Brush tops with egg wash if desired.

Bake for 20-25 minutes until golden brown.