Preheat your oven to 400°F (200°C) and prepare a baking sheet.

Pierce each potato several times with a fork and brush with melted butter. Sprinkle with kosher salt, wrap in foil, and roast for 45–60 minutes until fork-tender.

Allow the potatoes to cool for 10 minutes, then peel and shred into fluffy strands.

In a mixing bowl, combine Greek yogurt, shallots, kosher salt, white pepper, and cayenne (if using) and whisk until smooth.

Gently fold in the shredded cheddar and Parmesan into the mixture.

Add the shredded potatoes to the cheese mixture and fold until well coated. Transfer to a greased casserole dish.

Bake in the oven at 375°F (190°C) for 20–25 minutes until edges bubble and top is golden brown.

Switch to broil for an additional 3–5 minutes, watching closely for desired crispiness.