Coat bowl of slow cooker with cooking spray. Place onion and chopped garlic in bottom of slow cooker. Season chicken on both sides and arrange in bottom of cooker. Drizzle 1/2 the lemon over chicken and evenly spoon fire-roasted tomatoes, beans and sliced olives on top.
Cover and cook on HIGH for 6 hours or LOW for 8 hours. During last 30 minutes of cooking time, stir in rinsed beans.
Just before serving, stir in oregano (fresh is best!) and spoon over orzo or other favorite pasta.