Preheat oven to 375°F (190°C).
Season chicken breasts on both sides with salt and black pepper.
Heat vegetable oil in a large oven-safe skillet over medium-high heat.
Add chicken breasts and sear until golden brown on both sides, about 3-4 minutes per side. Remove chicken and set aside.
In the same skillet, reduce heat to medium and add butter.
Add sliced onions and mushrooms; sauté until softened and lightly browned, about 6-8 minutes.
Sprinkle flour over the vegetables and stir well to combine. Cook for 1-2 minutes to remove raw flour taste.
Gradually whisk in beef broth and add garlic powder, stirring constantly until gravy thickens.
Return chicken breasts to the skillet, spoon some gravy over them, and transfer the skillet to the preheated oven.
Bake uncovered for 15-18 minutes, or until chicken reaches an internal temperature of 165°F (74°C).
Remove from oven and let rest for a few minutes. Serve chicken centered on plates, smothered with mushroom and onion gravy, garnished with chopped parsley.