Heat a large cast iron pot over medium-high heat. Add olive oil and place the seasoned chicken thighs in the pot. Brown each thigh for about 3 minutes on both sides, then remove and set aside.

In the same pot, add chopped onions, celery, carrots, and red bell pepper. Sprinkle with salt and sauté for around 3 minutes. Add minced garlic and sliced mushrooms, cooking for an additional 4 minutes.

Stir in crushed tomatoes and tomato paste. Add rosemary, sage, garlic powder, salt, black pepper, and red pepper flakes, mixing well. Nestle browned chicken back into the pot.

Preheat oven to 350°F (175°C). Cover the pot with a lid and transfer to the oven. Bake for 50 minutes, then remove lid and bake for an additional 20 minutes.

Remove from oven, let it rest for a few minutes, and garnish with chopped basil before serving.