Pat dry the chicken thighs and season generously with salt and pepper. Heat olive oil in a skillet over medium-high heat. Once shimmering, sear the chicken thighs skin-side down until golden brown, about 5 minutes. Flip and sear the other side for another 3-4 minutes. Remove and set aside.
In the same skillet, add chopped onion and bell pepper. Sauté until fragrant and slightly softened, about 4-5 minutes, scraping up browned bits from the bottom with your spoon. Add minced garlic and cook for another minute until fragrant.
Transfer the seared chicken to the slow cooker. Pour the diced tomatoes along with their juices into the skillet to deglaze, scraping up any flavorful bits, then pour this mixture over the chicken. Add sliced olives, dried oregano, and a splash of red wine if using.
Top the chicken with the sautéed vegetables and sprinkle with chopped fresh basil (reserve some for garnish). Cover and cook on low for 6 hours or high for about 3 hours until the chicken is tender and falling off the bone.
Check the dish after a few hours — the sauce should be bubbling and thickened, and the chicken should be fall-apart tender. Taste the sauce and adjust salt, pepper, or herbs as needed.
Once ready, serve the chicken hot, spooning the rich sauce over it. Garnish with remaining chopped basil and enjoy this hearty, comforting Italian dish with pasta, rice, or crusty bread.
