Preheat oven to 350°F. Wash and peel the potatoes, adjust a mandoline slicer to 1/8, and slice the russet potatoes, sweet potatoes, yellow squash, and onions.
Alternate the slices into an 8"x10" baking dish and set aside.
Place a large skillet over med-low heat and add 3 tablespoons of unsalted.
When the butter is melted whisk in 3 tablespoons of all-purpose flour, whisk the mixture until the roux becomes a light blond color. Whisk in the half and half.
Add the grated white cheddar cheese a little at a time until it is fully incorporated.
Whisk in the sea salt, freshly cracked black peppercorns, onion powder, and smoked paprika.
Reduce the heat to low and allow the mixture to thicken until it coats the back of a spoon.
Pour the mixture over the sliced potatoes, squash, and onions.
Place the dish into the 350°F oven on the center rack, bake for one hour.
Remove the potato dish from the oven and evenly scatter on top the remaining grated pecorino cheese, finishing with chopped parsley.
Serve.