Preheat oven to 400 degrees and begin by making the cheese sauce. Chop and peel the russet potatoes, carrots and onion. Put in a pot with the 2 cups of water and bring to a boil. Reduce heat to a simmer and simmer until fork tender.
While veggies for cheese sauce are simmering, chop the collard greens and slice the yukon gold potatoes, mushrooms and onions and set aside. A mandolin works great for slicing but if you don't have one, you can slice by hand as well, it will just take a little longer.
When veggies for cheese sauce are fork tender, add the veggies, along with the water, to a high speed blender. Add the rest of the cheese sauce ingredients to the blender and blend until smooth and creamy.
Now, layer your potatoes in a 9x13 glass baking dish. Begin with a thin layer of cheese sauce. No oil is needed here, just the cheese sauce. Next, add a layer of potatoes, then add a layer of the mushrooms and onions, and finally, add a layer of the collard greens.
Next, add another layer of cheese sauce, then a layer of potatoes, another layer of cheese sauce, a layer of mushrooms and onions, and another layer of collard greens.
Finish the dish with a layer of potatoes and a final layer of cheese sauce. You may or may not use all of the cheese sauce. It just depends on how cheesy you'd like this potato dish to be. Leftover sauce can be used for Mac and Cheese.
Cover with aluminum foil and bake for 35 minutes. Remove foil and bake for another 15 minutes, until lightly browned.
Remove potatoes from oven and allow to sit for 5 minutes before serving.