Pat chicken thighs dry with paper towels. In a shallow dish, combine flour, 1/2 tsp salt, and 1/4 tsp black pepper. Dredge each chicken thigh in the flour mixture, shaking off excess.

Heat olive oil in a large skillet over medium-high heat. Sear chicken thighs for 3-4 minutes per side, until golden brown. You might need to do this in batches to avoid overcrowding the pan. The chicken doesn't need to be cooked through. Remove chicken from skillet and set aside.

In the same skillet, add chopped onion and bell peppers. Sauté for 5-7 minutes until softened. Add sliced mushrooms and continue to sauté for another 3-5 minutes until mushrooms release their liquid and begin to brown. Stir in minced garlic and cook for 1 minute more until fragrant.

Transfer the sautéed vegetables to the bottom of a 6-quart slow cooker. Place the seared chicken thighs on top of the vegetables.

In a medium bowl, whisk together crushed tomatoes, tomato paste, chicken broth, red wine vinegar, Italian seasoning, oregano, red pepper flakes (if using), 1 tsp salt, and 1/2 tsp black pepper. Pour the sauce evenly over the chicken and vegetables in the slow cooker.

Cover the slow cooker and cook on HIGH for 4 hours, or on LOW for 7-8 hours, until the chicken is very tender and easily shreds with a fork.

Once cooked, gently stir the chicken and sauce. Taste and adjust seasoning if needed. Garnish with fresh chopped parsley, if desired. Serve hot over pasta, polenta, rice, or with crusty bread.