Prepare the pan and coating. Preheat the oven to 375 degrees F. Butter generously the bottom and sides of a circular 1.75-quart or an 8-by-8-inch baking dish. In a food processor, add the pecans, brown sugar, sage, cinnamon and a pinch of salt. Pulse until crumbly like crushed cookies. Pour into the dish. Over a large bowl, tilt and tap the dish to coat all sides with the mixture. Add a bit more to the bottom of the dish to create a layer about 1/8-inch thin. Reserve the remaining mixture for the top. Refrigerate the coated dish and remaining topping until ready to fill.
Prepare the potatoes. Microwave the potatoes as instructed on the bag (usually 8 minutes). Once cooled to room temperature, break in half and place in a food processor with the butter, sugar, cream, flour, cinnamon and yolks. Pulse, and then blend until smooth. Taste and season with a pinch of salt if needed. Remove to a large bowl and fold in the egg whites. Pour into the prepared dish, smoothing out the top. Sprinkle the rest of the reserved topping evenly over the potatoes and gently press down to flatten.
Top and bake. Bake until the edges and top are golden, about 30 minutes. Raise the heat of the oven to 400 degrees F. Pour the marshmallows over the top and pat gently to an even layer. Bake while watching until the top is golden brown, 4 to 6 minutes. Serve immediately.