Pat the chicken thighs dry with paper towels and lightly season both sides with a pinch of salt and pepper.

Heat olive oil in a large, deep skillet or Dutch oven over medium-high heat. Add the chicken thighs, skin side down, and cook for 4-5 minutes on each side, until golden brown. Remove the chicken from the pan and set aside.

Reduce the heat to medium. Add the diced onion, garlic, mushrooms, celery, and carrot to the same pan. Stir occasionally, cooking for 5-7 minutes until the vegetables have softened and the onions are translucent.

Stir in the tomato paste and cook for 1-2 minutes to deepen its flavor. Then, pour in the crushed tomatoes and stir to combine.

Add the rosemary, sage, fresh basil, salt, pepper, and red pepper flakes. Nestle the browned chicken thighs back into the pan, making sure they're submerged in the sauce. Bring the mixture to a gentle simmer, then reduce the heat to low.

Cover with a lid and cook for 35-40 minutes, stirring occasionally. The chicken should be tender, and fully cooked.

Remove the rosemary and sage before serving. Plate the chicken with the sauce and garnish with freshly grated parmesan cheese and chopped parsley. Serve with pasta, rice, mashed potatoes or crusty bread. Enjoy!