Preheat oven to 350F.
Rub salt and pepper on both sides of chicken.
Heat oil in a skillet over medium-high heat. Add chicken and sear both sides until browned, about 2-3 minutes per side. Transfer out of pan and place in a baking dish.
Add onion to the same pan and cook until softened, about 4-5 minutes.
Add mushrooms and cook until softened, about 3-4 minutes. Transfer onion-mushroom mixture out of pan and place in baking dish on top of chicken.
Sprinkle rosemary on top. Bake until cooked through, about 30-40 minutes.
