Preheat the oven to 350 degrees F.
Toss the lamb with 2 Tb oil, garlic, Greek spice mix, salt and pepper. Marinate up to 24 hours covered in the fridge. Remove from fridge 15-30 min before cooking to help the meat come to room temperature for better browning.
In a Dutch oven (at least 4 qt), heat the remaining 2 Tb oil to medium high heat until shimmering. Add the lamb chops in an even layer to the hot pan. Brown on each side for 2-3 minutes. Set aside on a plate.
Turn the heat down to medium and add the onion to the pot. Add more oil if needed, if the pan is too dry. Saute the onion until softened, about 6-8 min. I sprinkle the onion with a little bit of salt to help release liquid.
Add the tomato passata to the pan along with the lamp chops and 1 c of boiling water. Season with salt.
Transfer pot to the oven, uncovered, and bake for 30 minutes. The tomato sauce will reduce and the lamb chops will get tender.
Remove the pot from the oven. Add the orzo around the lamb along with the remaining ¾ c boiling water. Stir so that the orzo is submerged with liquid. Return to the oven and cook for 15 minutes. Stir halfway through to make sure all the orzo is cooked through. Taste and add salt if needed.
Sprinkle with feta cheese and parsley to serve from the pot.