How to Make It
Preheat the oven to 375°. In a bowl, mix 2 tablespoons of the oil with the mustard, garlic and herbs. Coat the lamb with the mixture; season with salt and pepper.
In a roasting pan, toss the onion, celery and carrot with the remaining oil. Place the lamb fat side up on the vegetables. Roast for 1 hour and 40 minutes, until an instant-read thermometer inserted in the thickest part registers 130°. Let rest for 15 minutes.
Set the roasting pan over 2 burners. Pour in the wine and reduce over moderately high heat to 1/4 cup, scraping up the browned bits on the bottom. Stir in the tomato paste and simmer for 1 minute. Add the demiglace and water and simmer, stirring occasionally, until the sauce is reduced to 2 cups, about 5 minutes. Strain the sauce into a small saucepan, pressing on the solids. Season with salt and pepper and keep hot.
Carve the lamb and serve the slices with the red wine sauce.