Heat oven to 400° F.
Pat the chicken dry with paper towels. Season the chicken with 1/8 teaspoon of the salt and 1/8 teaspoon of the pepper.
Heat the oil in a medium ovenproof skillet over medium-high heat. Cook the chicken until browned, 2 to 3 minutes per side. Transfer to a plate.
Add the onions and mushrooms to the skillet and cook, stirring occasionally, for 5 minutes. Add the garlic, tomato, parsley, and the remaining salt and pepper. Cook for 2 minutes.
Return the chicken to the skillet and transfer to the oven. Roast until the chicken is cooked through, 8 to 10 minutes. Serve with brown rice (if using).