Pat chicken pieces dry with paper towels; season with salt and pepper.
In a large oven-safe skillet, heat olive oil and butter over medium-high heat. Add the chicken and cook until browned (5-7 minutes). Remove from the pan.
Sauté sliced onion for 3-5 minutes until softened; add minced garlic and red bell pepper, cooking for another 2-3 minutes.
Pour in diced tomatoes (undrained), chicken broth, oregano; stir to combine.
Return the chicken to the skillet, simmer covered for 15-20 minutes until cooked through.