Season the lamb all over with salt and pepper, then sprinkle on the rosemary. Allow the meat to marinate overnight, uncovered in the fridge. The next day, allow the lamb to come to room temperature for 30 minutes before cooking.
Preheat the oven to 250F. Heat the tallow in a heavy skillet, then brown the lamb deeply. Put the crusts of bread, some vegetables or something else in the pan to elevate the roast so the bottom of the pan doesn’t over cook the bottom of the roast. Alternately, cook the roast on a baking sheet lined with a roasting rack, which is ideal, and my preference for restaurant work.
Cook the roast for 30 minutes, or until a thermometer reads 125-130F. This temperature may seem rare, but trust me, it will be good. Cooking at a lower temperature means you get a perfect rose center, edge to edge.
Allow the roast to rest for at least 20 minutes, then reheat gently in the oven before slicing (always against the grain here) preferably putting the slices of meat on top of piping hot vegetables or something else on a plate to help them stay warm—a chefs plating trick. You will notice when slicing that the roast doesn’t bleed over the cutting board, and it’s perfectly pink and juicy.