Lightly coat the inside of a 3. 5-quart or 4-quart slow cooker with nonstick cooking spray.
In the prepared cooker, combine the undrained tomatoes, zucchini, yellow squash, barley, tomato paste, garlic, oregano, and pepper.
Pour the vegetable broth over the mixture and stir gently to combine.
Cover and cook on the low-heat setting for 6 to 8 hours or on the high-heat setting for 3 to 4 hours, until the barley is tender.
Stir in the frozen green beans.
Cover and let the stew stand for 10 minutes to allow the beans to heat through.
Ladle into bowls and sprinkle each serving with crumbled feta cheese before enjoying.