Step 1: Pat the boneless, skinless chicken thighs dry with paper towels. Season generously with salt, black pepper, and Italian seasoning. If you wish, briefly sear the chicken in a hot pan with a tablespoon of olive oil for 2-3 minutes per side until lightly browned. This step is optional but adds depth of flavor.
Step 2: In your slow cooker, combine the diced onion, sliced bell peppers, and minced garlic, spreading them evenly on the bottom.
Step 3: Place the seasoned chicken thighs on top of the vegetables in the slow cooker.
Step 4: In a medium bowl, whisk together the crushed tomatoes, tomato paste, chicken broth, red wine (if using), bay leaf, dried oregano, dried basil, and red pepper flakes.
Step 5: Pour the tomato mixture evenly over the chicken and vegetables in the slow cooker, ensuring the chicken is mostly submerged.
Step 6: Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is incredibly tender and easily shredded with a fork.
Step 7: Once cooked, remove and discard the bay leaf. Taste the sauce and adjust seasonings as needed. If the sauce is thinner than desired, you can remove the chicken, cover it, and simmer the sauce on the stovetop for a few minutes to reduce, or stir in a cornstarch slurry (1 tbsp cornstarch mixed with 2 tbsp cold water).
Step 8: Serve hot over your favorite pasta, polenta, rice, or with crusty bread. Garnish with fresh chopped parsley and/or grated Parmesan cheese.