Heat olive oil in a large skillet or Dutch oven over medium-high heat
Season chicken pieces with salt and pepper
Brown the chicken on both sides, then remove and set aside
Add chopped onion, bell pepper, and sliced mushrooms to the pan
Cook until softened
Add minced garlic and cook briefly
Stir in tomato paste
Add crushed tomatoes, chicken broth, dried oregano, dried basil, and red pepper flakes
Return the chicken to the pan
Add olives and capers if desired
Cover and simmer until the chicken is cooked through and tender
Adjust seasoning with salt and pepper
Garnish with chopped parsley
Serve hot over pasta, rice, or polenta