Preheat your oven to 350°F.
Heat canola oil in a large oven-proof skillet over medium heat. Add the chicken and diced onion; cook until the chicken is lightly browned on all sides, about 4-5 minutes (the chicken does not need to be cooked fully through at this point).
1 tablespoon canola oil, 1 ¼ pounds boneless skinless chicken breast, ½ cup onion
Add the butter and when melted sprinkle the flour over the chicken and onion, stirring constantly for 1–2 minutes.
3 tablespoons butter, ¼ cup all-purpose flour
Slowly pour in the chicken broth and milk while stirring continuously to prevent lumps.
1 can chicken broth, ½ cup milk
Stir in the poultry seasoning, garlic powder, salt, black pepper, and frozen mixed vegetables. Bring to a simmer and cook until the sauce begins to thicken, then remove from the heat.
1 bag frozen mixed vegetables, ½ teaspoon poultry seasoning, ½ teaspoon garlic powder, ½ teaspoon salt, ¼ teaspoon black pepper
Spoon the mashed potatoes in small dollops over the chicken mixture, then gently spread into an even layer to cover the top.
Bake uncovered for 25 minutes, or until the filling is hot and bubbly around the edges.
Remove from the oven and let stand for 5 minutes before serving to allow the sauce to thicken slightly.