1. Melt butter in a skillet over medium heat. Add the chopped onion and cook until translucent, about 3-4 minutes.

2. Sprinkle flour over the onions and cook, stirring constantly, for 1-2 minutes.

3. Slowly whisk in the chicken broth and milk, stirring until the sauce thickens.

4. Add shredded chicken and mixed vegetables. Season with salt and pepper. Simmer for 2-3 minutes. Remove from heat and cool slightly.

5. Preheat oven to 400°F (200°C). Roll out puff pastry and cut into 12 squares.

6. Press each square into a greased muffin tin well.

7. Spoon the filling evenly into each pastry-lined muffin cup.

8. Bake for 18-22 minutes or until golden brown and puffed.

9. Let cool for 5 minutes before removing from muffin tin.

10. Garnish with parsley if desired, and serve warm.