Heat 2 tablespoons olive oil in a large high-sided skillet or Dutch oven over medium heat. Add chicken thighs and sear on all sides until a golden crust forms, 4-5 minutes. Season generously with salt and pepper on all sides.

Remove the chicken thighs from the skillet and set aside on a plate.

In the same skillet, add onions and mushrooms and season to taste with salt and pepper. If necessary, add an additional 1 tablespoon of olive oil.

Cook until the onions become translucent, and the mushrooms soften, approximately 4-5 minutes.

Add garlic and bell pepper and continue cooking for another 2-3 minutes.

Add wine, crushed tomatoes with their juices, oregano, red pepper flakes, olives, and capers. Stir to combine.

Bring the sauce to a gentle simmer over medium-low heat.

Add the chicken and any accumulated juices back to the sauce and cook until the chicken is cooked through and the sauce is slightly thickened, approximately 30-35 minutes.

Remove from heat and stir in the fresh parsley. Taste and adjust the seasoning as desired. Serve over cooked spaghetti or polenta.