Sear the Chicken : Heat olive oil in a skillet over medium-high heat. Season chicken with salt and pepper. Sear chicken on both sides until golden brown, about 3–4 minutes per side. Remove and set aside.
Prepare the Crockpot : Place sliced peppers, onion, and garlic at the bottom of the slow cooker. Add diced tomatoes, tomato sauce, white wine, oregano, basil, red pepper flakes, salt, and pepper. Stir to combine.
Add Chicken : Place seared chicken on top of the vegetable and tomato mixture. Cover the slow cooker with the lid.
Cook : Cook on low for 6–7 hours or high for 3–4 hours until chicken is tender and cooked through.
Finish and Garnish : Taste the sauce and adjust seasoning if necessary. Sprinkle with fresh parsley before serving.