Preheat the oven to 400 degrees F.
Using your hands, rub the lamb all over with the olive oil. Pat the garlic and herbs evenly all over the surface of the meat. Season the meat with the salt and pepper and place the lamb in a roasting pan. Put the lamb in the oven and roast for 30 minutes. Reduce the oven temperature to 350 degrees F and continue to cook for about 1 hour longer for medium-rare, or until an instant-read thermometer inserted into the center of the roast registers about 145 degrees F. Be careful that the thermometer does not touch the bone. Allow to rest for 10 minutes before carving. Slice thinly around the bone and serve.
While the lamb is roasting, heat the Espagnole sauce in a saucepan over medium heat. Bring to a boil and continue to cook until the sauce is reduced in volume by 1/4. Stir in the Madeira and continue to simmer until sauce is thick enough to coat the back of a spoon, 6 to 8 minutes. Whisk in the butter and truffles. Reduce the heat to low and keep warm. (Do not allow the sauce to boil.)
Also while the lamb is roasting, combine the cream and cheese in a saucepan over medium heat. Stir until the cheese melts. Add the potatoes and mash well. Season with salt and white pepper to taste. Keep warm until ready to serve.
When the lamb has rested and is carved and ready to serve, spoon the potatoes in the center of each plate. Arrange the slices of lamb around the potatoes. Spoon the sauce around the potatoes. Garnish with the spinach, if desired.
In a stockpot, whisk the hot stock into the roux. In a large saute pan, heat the bacon fat. Add the vegetables, season with salt and pepper, and saute until wilted, about 5 minutes. Stir in the tomato puree and cook for about 5 minutes. Add the tomato/vegetable mixture to the stock/roux mixture. Add the bay leaf, thyme, and peppercorns and continue to simmer, skimming as needed, for about 45 minutes. Taste and adjust seasoning with salt and pepper. Strain the sauce through a fine mesh sieve and discard solids.