Preheat the oven to 400°F (200°C).
Cook the vegan chick’n pieces until golden; set aside.
Heat olive oil in a large skillet. Sauté onion, carrots, and celery until softened.
Add garlic, thyme, sage, salt, and pepper. Stir for 30 seconds.
Sprinkle flour over the vegetables and stir until coated.
Slowly pour in vegetable broth, stirring until thick and gravy-like.
Add plant milk and mixed veggies; simmer 3–5 minutes until creamy.
Fold in the cooked chick’n pieces and remove from heat.
Transfer the filling to a baking dish.
Place puff pastry on top; cut 2–3 steam vents.
Bake for 25–30 minutes, or until golden brown.
Let cool for 10 minutes before slicing.