1. Heat olive oil in a large skillet over medium heat. Add diced onion and cook until translucent, about 3–4 minutes. Stir in garlic and mushrooms, sautéing until browned and moisture has evaporated.

2. Sprinkle flour over the mushroom mixture and stir for 1 minute. Gradually add milk, stirring to create a smooth sauce. Add cream, mustard, thyme, salt, and pepper. Simmer until thickened, then add chicken. Mix and cook for 2 more minutes. Let cool completely.

3. Preheat oven to 400°F (200°C). Roll out puff pastry slightly and cut into equal rectangles. Spoon filling onto half, leaving a border. Cover with the remaining pastry and seal edges. Crimp with a fork.

4. Place on a parchment-lined baking sheet. Brush with beaten egg and score tops for decoration.

5. Bake for 20–25 minutes or until golden and puffed. Let cool slightly. Garnish with herbs if desired and serve warm.