1 tablespoon olive oil
1 1/2 pounds boneless, skinless chicken breasts, cut into 1-inch pieces
1 medium onion, chopped
1 (16-ounce) bag frozen peas and carrots
2 cloves garlic, minced
2 cups low-sodium chicken broth
3 tablespoons all-purpose flour
1/2 cup whole milk
Freshly ground black pepper
1 store-bought pie crust, thawed if frozen
Preheat the oven to 400°F.
Heat olive oil in a 9 or 10-inch cast-iron skillet over medium-high heat. Add the chicken, season with salt and pepper, and sauté until cooked through, 5 to 6 minutes. Transfer the chicken to a plate.
Add the onion to the skillet. Sauté until tender and lightly browned, 3 to 4 minutes. Add the peas and carrots and garlic and cook 1 minute more. Sprinkle in the flour and stir until the flour dissolves. Add the chicken broth, bring to a simmer, and stir until thickened, 8 to 10 minutes. Turn off the heat, stir in the milk, and season as needed with salt and pepper.
Add the chicken to the skillet and stir to combine. Unfold the pie crust and place over the top of the filling, tucking the excess edges inside the pan. Use a sharp knife to cut 4 (1-inch) slits in top of the crust to vent. Bake until the crust is golden-brown, about 20 minutes.