Rinse the chicken and pat it dry with paper towels. Cut it crosswise into 8 equal pieces. With the cut side up, use the palm of your hand to flatten each piece to slightly less than 1/2-inch thickness. Sprinkle both sides of the chicken pieces with some of the rosemary, garlic powder, salt, and black pepper, and set aside.
Coat a large nonstick skillet with the olive oil and preheat over medium-high heat. Add the chicken and cook for about 2-3 minutes on each side or until nicely browned and no longer pink inside. Remove the chicken from the skillet and set aside to keep warm.
Add the onion rings, mushrooms, and 1 tablespoon of the broth to the skillet. Cover and cook, stirring frequently, for a couple of minutes or until the onions and mushrooms start to brown and begin to soften. Add a little more broth if the skillet becomes too dry, but only enough to prevent scorching.
Add the remaining broth, wine, and tomatoes to the skillet mixture and bring to a boil. Reduce the heat to medium-low, cover, and cook for about 3 minutes or until the tomatoes have softened. Raise the heat to medium-high and cook uncovered, stirring frequently, for several minutes or until about 1/4-1/3 cup of liquid remains in the skillet.
To serve, place some of the chicken on each of 4 serving plates. Top each serving with some of the vegetable mixture, pan juices, and a sprinkling of parsley. Serve hot.