One of the things I like best about this vegan version of shepherds pie is that it tastes like an indulgence without actually being one. Each warm, cozy serving is rich with exotic, complex and decadent truffle flavor but only has about 550 calories (that's for three servings per recipe - which is a nice, heaping helping!) and, of course, no cholesterol. If vegan isn't your thing, you can easily substitute regular milk and butter in the mashed potatoes, but do give hazelnut milk a try. It is a mild, creamy substitute for dairy that does not impart that rather unpleasant "beany" taste that soy milk tends to have.
Bring a large pot of water to boil on high heat. Peel potatoes and add to water. Boil until potatoes are fork-tender, about 40 minutes. Reserve 1 cup of cooking liquid for gravy.
Meanwhile, in a large saute pan over medium-high heat, add 2 T olive oil. When oil is hot, add leeks and cook until slightly caramelized, about 6 minutes. Add mushrooms, carrots and peas and saute for an additional 5 minutes, adding a little more olive oil if necessary. Remove vegetables from pan and set aside.
Return pan to medium-high heat and add 2 T of olive oil. Add flour and whisk until combined. Switch to a wooden spoon and scrape the caramelized vegetable bits off the bottom of the pan. Lower the heat to medium and carefully drizzle 1 cup of potato cooking liquid into the pan, stirring constantly. Slowly add 1 cup hazelnut milk and stir well. Add vegetable broth powder, rosemary and salt and pepper to taste. Cook until desired thickness is achieved, about 5 minutes. Add vegetables to gravy and stir well to combine.
Assemble mashed potatoes. Drain cooked potatoes and press through a ricer into a medium-sized bowl. Add the rest of the ingredients and lightly fold in with a spatula. Adjust salt and pepper to taste.
Preheat oven to 375. In a small casserole dish (I used a 1 3/4 quart dish that measured 8 1/2 by 6 1/2 inches), add a little olive oil and rub with a paper towel to coat the inside. Add the vegetables and gravy first, then layer the mashed potatoes on top. Bake shepherds pie until the potatoes turn lightly golden, about 30 minutes. Allow to cool for at least 10 minutes before serving.