Preheat the oven to 325 degrees Fahrenheit.

Drizzle olive oil over the lamb and season with salt and pepper.

Prepare the roasting tray with a splash of water and roast the lamb for four hours.

Remove the lamb from the roasting tray.

Lift the lamb and place it on the surface. Chop the meat and keep aside the bones.

Clean the roasting tray by picking up any sticky bites from the bottom of the tray.

Splash boiling water on the tray and stir around. Preserve it in the glass jar.

Place the diced onions, carrots, celery, and swede in a pan. Heat the pan on medium flame with adding lamb fat. Stir in rosemary leaves and cook for twenty minutes.

Stir flour, lamb, bones, and tray juices in the saucepan. Add 1.5 liters of water and cook well.

Bring the mixture to a boil over high heat and cook on a low flame for forty minutes.

Discard the bones.

Place the sieve over the pan and spoon the lamb stew into the batches.

Transfer everything to a separate bowl.

Chop the potatoes and boil them in the salted water. Cook for fifteen minutes.

Drain the potatoes and let them cool. Then add butter and cheddar cheese and season with salt and pepper. Mash the potato.

Preheat the oven to 400 degrees Fahrenheit.

Place one tablespoon of lamb fat in the bottom of the pie dish. Sprinkle half of the bread crumbs and rosemary leaves on top.

Now layer the mashed potatoes into the pie dish. Spoon the filling and the gravy.

Push down the filling into the pie dish. Now place the leftover mashed potatoes and pat them down.

Top with grated cheese and breadcrumbs. Drizzle oil on the top.

Bake for one hour and ten minutes.

Jamie Oliver's shepherd's pie is ready to serve.