Season chicken with salt and pepper to taste. Cook in a large skillet over medium-high heat 1-2 minutes on each side til browned. Transfer to greased slow cooker.
Add onions, olives, red peppers, and capers to slow cooker (tuck them around the sides so they aren't covering up the chicken). Whisk together italian seasoning, lemon juice, and garlic and pour over chicken.
Cover and cook on low for 4 hours or on high for 2 hours. Garnish with fresh thyme or oregano and serve.