Add the chicken pieces, artichoke hearts, onion, carrots, bell pepper, and garlic to the slow cooker.
Pour in the diced tomatoes with their juices and the chicken broth.
Sprinkle in the oregano, thyme, salt, and black pepper. Stir gently to combine.
Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours, until the chicken is cooked through and the vegetables are tender.
Stir in the spinach during the last 10 minutes of cooking so it wilts but stays bright.
Turn off the heat and stir in the lemon juice just before serving.