Preheat the oven to 350 degrees F (175 degrees C). Spray a baking sheet with cooking spray.
Pour mixed vegetables into a microwave-safe bowl. Add water. Heat in the microwave until warmed through, 1 to 3 minutes. Drain.
Drizzle a skillet with vegetable oil and heat over medium heat. Add onion and cook until soft, about 3 minutes. Add mixed vegetables, chopped chicken, condensed soup, salt, and pepper. Cook and stir until filling is heated through, 3 to 5 minutes more.
Open biscuits and flatten each circle out with your fingertips or a floured rolling pin. Wet edges of biscuit with water using a thin brush. Place a portion of filling on one half of a circle and fold the other half over. Press edges with a fork to seal closed. Place mini pie carefully onto the prepared baking sheet. Repeat with remaining biscuits and filling.
Bake in the preheated oven until the outsides are golden, 30 to 35 minutes.