01 - Lightly season the chicken thighs with salt and pepper on all sides.
02 - In a large skillet over medium-high heat, sear the chicken thighs for 2-3 minutes per side until lightly browned. This optional step adds extra flavor to the final dish.
03 - Place the sliced onions, bell peppers, mushrooms, and garlic in the bottom of the slow cooker, spreading them evenly.
04 - Arrange the browned chicken thighs on top of the vegetable layer in the slow cooker.
05 - Pour in the diced tomatoes, crushed tomatoes, and white wine (or chicken broth). Add the oregano, basil, thyme, salt, black pepper, and crushed red pepper flakes. Stir gently to distribute the seasonings.
06 - Scatter the capers and black olives evenly over the top of the mixture.
07 - Cover and cook on LOW for 6 hours or on HIGH for 3 hours, until the chicken is tender and pulls away easily from the bone, and the sauce has thickened.
08 - Taste the sauce and adjust salt, pepper, or herbs if needed before serving.
09 - Sprinkle with chopped fresh parsley and serve hot. Optionally top with freshly grated Parmesan cheese. Pair with crusty bread, polenta, or gluten-free pasta if desired.