In a large skillet or Dutch oven, heat the olive oil over medium-high heat.
Season the chicken thighs with salt and pepper. Once the oil is hot, add the chicken thighs skin-side down. Sear for about 5-7 minutes on each side, or until golden brown. Remove from the pan and set aside.
In the same skillet, add the chopped onion and sliced bell pepper. Sauté for about 5 minutes until the vegetables are softened.
Add the minced garlic and cook for an additional 1 minute until fragrant.
Stir in the diced tomatoes, dried oregano, dried basil, and red pepper flakes (if using). Mix well.
Pour in the chicken broth and bring the mixture to a simmer.
Return the seared chicken thighs to the pan, skin-side up, and cover. Reduce the heat to low and let it simmer for 30 minutes.
Occasionally check the chicken and stir the sauce. If it becomes too thick, you can add a bit more chicken broth or water.
Once the chicken is tender and cooked through, taste the sauce and adjust the seasoning with more salt and pepper if needed.
Garnish with freshly chopped parsley before serving.