In a medium pan over medium heat, add the oil and saute the onions until translucent. Add the tomatoes to the pan and squash them to form liquid. Bring the mixture to a boil, then reduce the heat to a simmer. Add the chicken breasts, broccoli, red and yellow bell peppers, mushrooms, bay leaves and thyme. Season with salt and pepper. Cook until the chicken is cooked through, 8 to 15 minutes depending on thickness.