Place a large pot over a low heat. Wash the lamb thoroughly, place it in the pot, add enough water to cover the lamb, the allspice, bay leaves, and rosemary. Season with salt and pepper and boil until the lamb is soft, skimming every now and then.
Preheat the oven to 200 °C (390 °F) on fan mode.
Once the lamb is ready, retain 400 ml (1 1/2 cups) of the broth and place the lamb in an ovenproof dish, big enough for it all to fit nicely.
In a bowl, mix the eggs, yogurt, flour and the retained broth very well with a hand whisk until the mixture is even.
Add the orzo, season with salt and pepper, stir well and pour the mixture over the meat.
Bake for 50-60 minutes and then serve.