In a large bowl, combine the chicken breast, salt, pepper, and chili powder and mix well.
Heat the vegetable oil in a large skillet, and add the seasoned chicken to the skillet in a single later. Cook for 7-10 minutes over medium heat, stirring every few minutes.
Add the sliced bell peppers, sliced onion, and minced garlic cloves, and stir well. Add the diced tomatoes (including the liquid - see notes!) and stir. Cover with a lid and allow to stew for about 5 minutes, or until the peppers are cooked to your liking and the chicken is fully cooked. Stir well and remove from heat. Serve hot, spooning the extra tomato sauce over pasta, rice, polenta, or your favorite side dish. Top with freshly grated Parmesan.
You can use one or two cans of diced tomatoes - your choice. Use two cans if you would like extra sauce to spoon over your side dish, or use just one can if you are serving this with a side that doesn't need extra sauce.