Fillet the lamb into a 3/4-1 inch flat piece. Rub each side with half the mustard, oregano, parsley, thyme, garlic, salt, pepper, and aleppo pepper. Roll the lamb up fat side out and tie with butcher's twine. Let sit at room temperature for 30-60 minutes. Preheat the oven to 450 degrees. Place the lamb on a baking tray and cook in the oven for 30 minutes. Reduce the heat to 350 and continue cooking until it reaches the desired temperature. 20 minutes for medium-rare 135 degrees, 28-30 minutes for medium 145 degrees, etc. Remove from the oven and rest for 10-20 minutes. Remove the butcher's twine, slice into 1/2 inch pieces, and serve with chimichurri.