Crust:

1. Place the almond flour, cassava flour, and guar gum into a food processor bowl. Pulse a few times to mix well.
2. Add the cold butter cubes and pulse until the mixture looks like coarse, dry crumbs. 
3. Add egg yolk and water and pulse 4-5 times. The dough will look crumbly but will hold together when you work with it.
4. Gather the dough into a ball on parchment paper. Flatten to a large disk shape. Roll between 2 paper sheets into a circle.
5. Peel the top sheet of paper from the dough and carefully flip the dough on the remaining sheet over the pie plate. Gently peel the paper and press the dough with your hands—trim edges with a knife.
6. Pre-bake the crust at 350°F for about 15 minutes or until lightly golden. Pierce it with a fork to prevent it from bubbling up while baking.

Filling:

1. For the filling, cut the leek in two lengthwise. Wash to remove any soil between the leaves. Chop into 1/3 inch stripes.
2. Heat olive oil in a large skillet over medium heat. Add leeks and cook until they are wilted, about 5-7 minutes. Mix in chopped sun-dried tomatoes. 
3. Distribute leeks and sun-dried tomatoes evenly across the crust. Top the quiche with feta cheese. 
4. In a mixing bowl, whisk together cream, eggs, salt, and pepper. Pour the mixture over the entire quiche.
5. Bake 30 - 40 minutes until the center is firm and the crust and top are nicely browned.