Preheat your oven to 375°F (190°C).

Combine flour and salt in a mixing bowl, then cut in the cold unsalted butter until it resembles coarse crumbs.

Beat the large egg and mix in the chilled water. Add this to the flour mixture, folding until it forms a dough ball. Refrigerate for about 15 minutes.

Sauté the chopped onions in a little butter over medium heat until translucent, about 5 minutes. Add the shredded cabbage and cook until wilted, around 3-5 minutes.

Stir in the sharp cheddar cheese, full-fat sour cream, salt, and pepper into the cabbage mixture. Let it cool slightly.

Roll out the dough on a floured surface until about 1/8-inch thick, place it gently into a pie dish, and fill it with the cabbage mixture.

Sprinkle additional cheese on top if desired, and bake for 40-45 minutes, or until the top is golden brown and bubbly.