Heat the olive oil in a large pot over medium heat. Add the lamb chunks and cook until browned on all sides, about 6 to 8 minutes.
Add the diced onion and minced garlic to the pot. Stir and cook for 3 minutes until the onion softens.
Pour in the chicken broth, lemon zest, lemon juice, salt, and black pepper. Bring the mixture to a boil, then reduce heat to low, cover, and simmer for 45 minutes until the lamb is tender.
Stir in the orzo and cook uncovered for 10 minutes, stirring once or twice, until the orzo is soft.
Remove the pot from heat and stir in the chopped fresh dill right before serving.