Truffle Umami Blast Burgers

Once potatoes have roasted 15 minutes, remove from oven and push to one side of sheet. Mound Parmesan on empty side in two even piles. (For 4, leave potatoes roasting and add cheese to a second sheet; bake on middle rack.) Return to oven and bake until cheese is melted and crispy at edges and potatoes are tender, 5-7 minutes. Let frico cool for 1 minute on sheet, then transfer to a plate.

Meanwhile, trim and thinly slice mushrooms. Heat a large drizzle of oil in a large, preferably nonstick, pan over mediumhigh heat. Add mushrooms; cook until golden brown, 3-4 minutes. Add remaining shallot; cook, stirring, until softened, 2-4 minutes. Stir in ¼ cup water (⅓ cup for 4 servings) and stock concentrate; cook until liquid has mostly evaporated, 1-2 minutes. Season with salt and pepper. Turn off heat; transfer to a plate. Wipe out pan.

Meanwhile, halve buns. In a medium bowl, combine beef and ¼ tsp truffle zest (½ tsp for 4 servings); season generously with salt and pepper. Form into two equal-sized patties (four for 4), each a bit wider than a burger bun.

Heat a drizzle of olive oil in pan used to cook mushrooms over medium-high heat. Add patties and cook to desired doneness, 3-5 minutes per side. Meanwhile, in a second small bowl, combine mayonnaise, sour cream, ¼ tsp truffle zest (½ tsp for 4 servings), salt, and pepper. (Taste and add more truffle zest if desired.) Toast buns until golden.