Preheat the oven to 350 degrees.

Wash and remove any bad spots from the potatoes, then cut them into quarters. Place them into a stockpot, and fill with water until just covering the potatoes.

Add 2 tablespoons of kosher salt and cook over high heat for 20 to 25 minutes, or until the potatoes are soft and can be pierced with a fork. Drain off the water and place the potatoes into a large mixing bowl.

Peel the butternut squash and cut it into one-inch cubes. Place the squash in a steamer pot with 1-2 cups of water over medium-high heat, and steam for 4 to 5 minutes, or until you can pierce the squash with the tines of a fork. Drain well. Add the squash to the potatoes.

Add the cheeses, heavy whipping cream, sour cream, and melted butter to the potato and squash, then use a wand mixer to mash until well combined. Note- this is the time to taste the squash and adjust to taste before adding the eggs.

Next, add the two whole eggs and two egg whites and mix thoroughly. Add the smoked flaky sea salt and freshly cracked black peppercorns.

Spoon the mixture into a lightly greased 9x13 casserole dish and bake on the center rack for an hour; the top should be golden brown.

Remove from the oven and serve.