Season chicken with desired amount of salt and pepper. In a large skillet, cook chicken over medium-high heat 1-2 minutes on each side until browned. Transfer to a greased slow cooker.
Add onions, olives and red peppers to slow cooker. Tuck these around the sides of the chicken so the chicken isn’t covered.
Whisk together lemon juice, garlic and Italian seasoning; pour over chicken.
Cover and cook on low 4 hours or on high 2 hours. Sprinkle with feta cheese and garnish with fresh basil, if desired.