Place the lamb stew meat, diced onion, and minced garlic into the slow cooker.
Pour in the beef broth, then add the lemon zest, lemon juice, dried oregano, salt, and black pepper.
Stir once to combine, cover, and cook on low for 8 hours until the lamb is very tender.
Stir in the orzo, replace the lid, and cook on high for 25 minutes until the pasta is soft.
Ladle into bowls and top each serving with crumbled feta cheese.