Heat 2 tablespoons of vegetable oil in a large skillet over medium-low heat. Add 1 large sliced onion and cook slowly for 20–30 minutes, stirring occasionally, until the onions are tender and golden brown. Set the onions aside.

In the same skillet, melt 2 tablespoons of unsalted butter over medium heat. Add 8 ounces of baby Bella mushrooms and sauté for 5–7 minutes until browned. Stir in 1 cubed Granny Smith apple and 2 tablespoons of granulated sugar, cooking for 5–8 minutes until tender. Combine with the caramelized onions.

In a mixing bowl, combine the onion and mushroom-apple mixture with 1 cup of grated Gruyère cheese, 2 tablespoons of minced chives, and 1 teaspoon of dried thyme. Mix gently until evenly distributed.

Unroll 1 sheet of thawed puff pastry on a lightly floured surface. Cut into 2-inch squares. Place about one tablespoon of filling in the center of each square, leaving a small border.

Pinch the pastry edges together to create parcels. Arrange on a parchment-lined baking sheet. Preheat oven to 400°F (200°C). Brush with a beaten egg wash. Bake for 15–20 minutes or until puffed and golden.