Set the Instant Pot to sauté and add the lamb pieces. Brown them for 4 to 5 minutes, stirring a few times so they color on all sides.
Add the diced onion and minced garlic. Cook for 2 minutes until the onion starts to soften.
Pour in the chicken broth, then stir in the lemon zest, lemon juice, oregano, salt, and pepper. Scrape the bottom to loosen any browned bits.
Secure the lid, set the valve to sealing, and cook on manual high pressure for 25 minutes. Allow a 10-minute natural release, then quick release any remaining pressure.
Open the lid and switch back to sauté. Stir in the orzo and let it simmer for 6 to 7 minutes, stirring once or twice, until the orzo is tender.
Turn off the pot and stir in the chopped dill. Ladle into bowls and top each serving with crumbled feta.