Preheat the oven to 400°F (200°C). Lightly grease a 12-cup muffin tin.

In a skillet, melt the butter over medium heat (or heat 1 tbsp olive oil). Add chopped onion and celery and cook 4–5 minutes until soft and translucent.

Stir in the flour and garlic powder; cook for 1 minute, stirring, to remove the raw flour taste.

Slowly whisk in the chicken broth and milk. Add Dijon mustard and thyme. Simmer for 4–6 minutes until the sauce thickens to a saucy gravy.

Stir in the cooked chicken and frozen mixed vegetables. Cook for 2–3 minutes until heated through. Taste and adjust salt and pepper. Remove from heat and let cool for 5 minutes.

Roll out puff pastry and cut 12 circles large enough to line the muffin cups (or use store-bought mini crusts).

Press pastry into each muffin cup, leaving edges overhanging slightly.

Spoon the chicken mixture into each pastry-lined cup (about 3 tablespoons each), leaving room at the top for a lid. Place a pastry circle or small pastry star on top and crimp edges.

Brush with egg wash if using.

Bake for 18–22 minutes until pastry is puffed and golden brown. If using biscuit dough, baking time may be 16–20 minutes—check for golden color.

Let cool in the tin for 8–10 minutes before removing. Garnish with chopped parsley and serve warm.