Bring a medium-size stockpot of water to a boil over medium heat and add the diced potatoes. Cook until fork tender.
Drain the potatoes and add to large mixing bowl. Add the buttermilk, 3 tablespoons butter, white truffle oil, 2 teaspoons salt and 1 teaspoon freshly ground black pepper. Whip until the potatoes are mashed. Set aside.
Preheat the oven to 350 degrees F.
Remove the sausages from the casing and combine it with the beef in a medium bowl. In a large saute pan, add 3 tablespoons of butter or oil. Add the beef and sausage mixture, the rosemary, and the remaining salt and pepper. Cook over medium heat until well browned. Remove the mixture from the pan and set aside. In the same pan add the carrots, celery, remaining 3 tablespoons of butter or oil and the maple syrup. Cook over medium heat for about 10 minutes. You don't want soggy vegetables; if anything, you want them still pretty crunchy. Remove from pan from the heat and set aside.
You can cook the pie in individual ramekins or bake it in a large glass casserole dish. Either way add the vegetables for the first layer, then the meat, and finish it off with the whipped potatoes, being careful to keep your layers from getting jumbled. Sprinkle with cheese and bake for 15 minutes for individual servings or 25 minutes for a casserole dish.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.