Mix all rub ingredients to form a paste. Spread rub mixture evenly over roast, cover tightly with plastic wrap and refrigerate overnight.
Preheat oven to 375°F. Discard plastic wrap and season lamb leg with kosher salt and ground black pepper. Bring to room temperature, about 40 minutes.
Mix carrots, onions, garlic and fresh rosemary sprigs and place on sheet pan.
Place lamb roast on top of veggies and roast for 1 hour, or until meat thermometer inserted into thickest part reads 125°F.
Heat beef stock in large saucepan. Rest lamb leg on wire rack while making sauce.
Place roasted vegetables and rosemary into saucepan with beef stock, and simmer for 2 minutes.
Remove saucepan from heat, discard rosemary sprigs before pureeing with immersion or stand blender.
Thinly slice rested lamb and platter. Garnish platter with fresh rosemary and pomegranate seeds and serve with sauce.