01 - Lightly season the chicken thighs with salt and pepper on all sides.
02 - Heat a large skillet over medium-high heat. Sear the chicken thighs for 2-3 minutes per side until lightly browned. This step is optional but adds extra flavor depth.
03 - Place the sliced onions, bell peppers, mushrooms, and garlic in the bottom of the slow cooker, creating an even bed for the chicken.
04 - Arrange the browned chicken thighs on top of the vegetable layer in a single layer.
05 - Add the diced tomatoes, crushed tomatoes, white wine (or broth), oregano, basil, thyme, salt, black pepper, and crushed red pepper flakes. Stir gently to distribute the seasonings evenly.
06 - Scatter the capers and black olives over the top of the mixture.
07 - Cover and cook on LOW setting for 6 hours or on HIGH setting for 3 hours, until the chicken is fork-tender and the sauce has developed a rich, thick consistency.
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. Pairs well with crusty bread, polenta, or pasta.