Preheat oven to 400 degrees F (200 degrees C) and begin to prepare vegetables and potatoes.
Peel the Russet Potato (3) and cut into long slices 1/8-inch thick lengthwise using a meat slicer or mandolin. Add the slices to a bowl of water to avoid browning. When finished, drain from the water and lay on a baking sheet greased with olive oil, avoiding overlapping. If needed, use two baking sheets or do this in two batches.
Drizzle Olive Oil (1 tablespoon) to glisten the potatoes, then sprinkle one side with Salt and Pepper (to taste). Bake until just tender, about 15 minutes. Set aside until needed.
Add the Asparagus (1 bunch) and Button Mushroom (8 ounce) to a baking sheet and drizzle with Olive Oil (2 tablespoon). Season with more salt and pepper to taste, then bake for 10 minutes. This par-cooks the vegetables before baking with the quiche and can be done a day in advance. Set aside until needed.
Prep the Leek (1) by trimming off the white stem-end and the tough green leaves. Remove the outer layer if needed. Cut the leek in half lengthwise, then cut into small slices. Transfer to a bowl of water to remove any grit and dirt. Rinse and dry.
Heat the Butter (2 tablespoon) in a small saute pan over medium-high heat. Saute the leeks until tender, about 10-12 minutes. Season with salt and pepper to taste and add to a bowl with the roasted vegetables. This can also be done a day in advance.
Reduce the oven to 350 degrees F (180 degrees C).
Grease a 12-inch round baking pan or cast-iron skillet with olive oil or cooking spray. Start by covering the rim of the pan or skillet with potato slices, leaving 1-inch of potato hanging over the edge. Use the remaining slices to cover the bottom, filling any holes.
In a large bowl, mix the Heavy Cream (1 cup) and Egg (6). This will be super thick! Add the Gruyère Cheese (4 ounce), Fresh Parsley (2 tablespoon), cooked vegetables, and leeks. Combine. Season with salt, pepper, and Ground Nutmeg (1 pinch).
Pour into the potato-lined pan, folding over the extra 1-inch of potato to create the crust. Bake covered for 40 minutes.
Uncover and continue to bake for 15-20 minutes, until a knife inserted comes out clean. If desired, broil for a few minutes to brown the potatoes.