Peel the potatoes if desired. Then, use the 1/8" setting on a mandoline (or use a very sharp knife) and slice the potatoes into thin, even slices. (See recipe notes below for slicing tips.)
Fill a large pot with water and heat to boiling. Add the potato slices and cook for 3 minutes. The slices should be just fork tender, but not cooked through yet. (See recipe notes.)
Immediately drain in a colandar and rinse thoroughly with cold water to stop the cooking. (See recipe notes.) Pat the potato slices dry with a towel, and place them in a large mixing bowl.
To Prepare the Leeks
Slice the leeks width-wise into 1/8" rings. Fill a mixing bowl with water and add the leeks to the bowl. Stir with your hand to allow the dirt to fall to the bottom of the bowl, separating the leek segments into rings as you go. Carefully remove just the cleaned leek rings from the bowl, allowing the dirt behind, and set aside. (If desired, set aside a couple tablespoons of leek rings to top the casserole.)
In a small saute pan, heat a tablespoon or so of water on medium high heat. Add the leeks and saute until just tender, about 5 minutes. Add more water a little at a time if things begin to stick. Add the garlic and saute 2 more minutes. Add the leeks and garlic to the mixing bowl with the potato slices.
To Make the Cheese Sauce
Add your soaked cashews (or chickpeas, if using) and all of the other Cheddar Sauce ingredients to a high powered blender, and process until thick and creamy. Scrape down the sides as needed. Add the cheese sauce the to mixing bowl wtih the potatoes, garlic and leeks.
To Assemble the Casserole
Gently toss the cheese sauce, potatoes, garlic and leeks together, being careful not to break apart the potates. Make sure all of the ingredients are coated with cheese sauce. You may have to use your hands to separate the leek slices and distribute them through the bowl.
Line a 9" x 13" baking dish with parchment paper. (Not absolutely necessary, but much easier for clean up.)
Carefully spread out the potato slices in the baking dish. Use a rubber scraper to scrape any excess cheese sauce from the bowl and spread it on top of the potatoes.
Cover with aluminum foil and bake for 30-40 minutes at 350 degrees Fahrenheit, or until the potatoes are fork tender. Remove the foil, add the reserved leeks as garnish, and bake 10 more minutes.
Cashews or chickpeas. This recipe is delicious (and tested!) both ways. I personally prefer the cashew version for this holiday-friendly dish. But if you're dealing with a nut allergy or need to limit calories, the chickpea version is still very good.To slice potatoes with a mandoline. I found the best way to do this is to first trim cut each potato in half, then trim 3, 1/8" slices off the tip of each potato with my knife. That gave me a flat surface to attach the vegetable holder part of the mandoline, and a flat surface on the mandolin's blade.To slice potatoes with a knife. I prefer the mandoline because without thin even slices, this dish can take longer to cook and/or cook unevenly. But it is possible to do this with a very sharp knife. Just go slowly and be sure to keep checking the thickness of your potatoes to make sure close to 1/8" and even as you can get them. Pre-cooking the potatoes. This helps the dish cook more quickly and evenly. Note, I intentionally didn't say 'parboil' the potatoes because the water never comes to a boil again. That's ok--you really don't want them to get too soft, or they'll break apart when you toss them in the cheese sauce. It's also important to really rinse them well in cold water to stop the cooking. Toss them gently in the colander to make sure you get them all.