Lightly grease the inside of a 6-quart slow cooker with non-stick spray or a small amount of olive oil.
Place the chicken in the bottom of the slow cooker.
Layer the red onion, roasted red peppers, and Kalamata olives on top of the chicken.
In a small bowl, whisk together garlic, oregano, thyme, salt, pepper, red wine vinegar, chicken broth, and olive oil. Pour over the chicken and vegetables.
Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the chicken is tender and cooked through.
About 15 minutes before serving, stir in the baby spinach and let it wilt in the heat.
Squeeze fresh lemon juice over the chicken and stir gently.
Serve warm, topped with optional feta cheese, parsley, and lemon wedges if desired.