Liberally season the chicken thighs with salt and pepper.
In a 4-quart high-walled skillet, heat 1 tablespoon of olive oil over medium-high heat. Add the chicken and cook 3-4 minutes per side, until lightly browned. Transfer to a bowl.
Add another tablespoon of olive oil to the pan, then add the mushrooms. Lightly season with salt and pepper, then cook until mushrooms begin to brown, about 5 minutes. Transfer the mushrooms to the bowl with the chicken.
Add the remaining tablespoon of olive oil to the pan, then add the onion and red bell pepper and season with salt and pepper. Cook over medium heat until the onion and pepper begin to brown, 5–7 minutes.
Create a well in the middle of pan and add the tomato paste, then cook for about 1 minute before mixing together with the onions and peppers.
Add the garlic, oregano, red pepper flakes, and rosemary and stir to combine before adding the red wine. Use the moisture from the red wine to scrape up any stuck-on bits from the bottom of the pan. Cook until the wine is reduced by half.
Add the diced tomatoes and chicken stock and bring to a simmer.
Place the chicken and mushrooms on top of the simmering sauce, pouring in any accumulated juices from the bowl. Sprinkle with the parsley.
Cover and cook for12 minutes, or until the internal temperature of the chicken reaches 165°F (75°C).
Serve chicken and sauce over pasta or with bread alongside.