Roasted Tenderloin of Pork with Cremini Mushroom Sauce

This is easy to make and fairly low in fat, believe it or not. Preheat your oven to 350 degrees. Place three medium to large washed sweet potatoes on a rack in the middle of the oven. Thinly slice two to three large shallots into thin rings. Seperate the rings and place in the middle of a sheet pan. Drizzle the shallots with a little olive oil and salt them, then toss them around to coat them and spread them out in a single layer. Place the sheet pan on the top rack of the oven. Check the shallots after 15 minutes and stir around again. Keep cooking them and checking until they are a nice deep brown color and then pull out and set aside.
Rub a pork tenderloin with sage, salt and pepper. In a cast iron skillet, or something similar, heat one tablespoon of olive oil to very hot and place tenderloin inside. Let each side get good brown color on it and then remove and place on an oven proof dish and stick in the oven. Pour one cup of white wine in the still hot skillet and let reduce by half, then add 1 cup of chicken stock or 1/2 a cup of veal demi-glace (look in the frozen section of a upscale food store, such as Whole Foods). Let reduce again until it becomes slightly thick, season with salt and pepper and then remove from heat. Chop two cups of cremini mushrooms into thin slices. Smash two cloves of fresh garlic. In a non stick pan heat one tablespoon of olive oil to a very high heat and toss in the mushrooms and garlic and saute until browned. Pour mushrooms into the pan with the sauce. Check on the pork roast after 20 minutes and use a thermometer to check the internal temperature. As soon as the roast reaches an internal temperature of 160 degrees remove from the oven and let rest at room temperature. Check the sweet potatoes (they take about an hour). When they are very soft to the touch remove them from oven. Cut open the sweet potato and scoop the insides out into a bowl and discard the skins. Add a small pat of butter and salt and smash with a fork. Turn the heat under the sauce back on (if its too reduced just add a little more chicken stock) and as soon as it's close to boiling turn the heat off and stir in one tablespoon of butter. Keep the butter moving around until it melts completely, this will make the sauce get slightly creamy.
To plate-
Slice the tenderloin on a slight diagonal. Place a mound of sweet potato puree on the plate. Place 3 to 5 slices of pork roast, depending on size of roast, leaning a little against the sweet potato. Spoon sauce and mushrooms out over the pork. Sprinkle a couple of tablespoons of the shallot rings over the sweet potato.