Preheat oven to 375°F. Place chicken breasts in a casserole dish.
Mix together chili powder and salt and sprinkle over top of chicken breasts.
Spread onions and peppers over the chicken breasts.
In a medium sized bowl, mix together diced tomatoes, tomato paste, green chilies, salsa, garlic and cumin. Pour over the chicken breasts.
Cover loosely with aluminum foil. Bake for 1 hour. Remove from oven and stir and check chicken for doneness with a meat thermometer. Cook an additional 10 to 15 minutes, if needed.
To serve, spoon extra sauce over top of chicken. Top with green onions, if desired.