In a Dutch oven or skillet, heat oil over high heat until hot. Add chicken; cook until brown on both sides, turning once, about 3 minutes for each side. Remove chicken from pot. Add mushrooms, onions and garlic. Cook and stir until mushrooms are golden, about 10 minutes. Return chicken to skillet. Stir in thyme and pepper. Add tomatoes with juice from can; bring to a boil. Reduce heat to medium low; cover and simmer until chicken is tender, about 10 minutes, stirring occasionally. Serve over steamed rice, if desired.