Peel and dice 2 large Russet potatoes. Place them in a pot of salted water and bring to a boil. Cook for 15-20 minutes, or until tender. Drain and keep warm.

Mash the warm potatoes until smooth. Add 1 cup of sharp cheddar cheese, ½ cup of sour cream, and 2 tablespoons of softened butter. Mix until combined.

In a skillet, melt 1 tablespoon of butter over medium heat. Add 1 finely chopped onion, and sauté until translucent, about 5 minutes. Stir in 2 minced garlic cloves and 1 teaspoon of fresh thyme, cooking for an additional minute.

Fold the sautéed onion and garlic into the potato mixture. Season with salt and pepper to taste.

Preheat the oven to 375°F (190°C). Roll out the pie crust and fit it into a 9-inch pie dish. Spoon the potato mixture into the crust.

Beat the egg and brush it over the crust. Bake for 30-35 minutes until the crust is golden and the filling is bubbling.

Remove from oven and let cool for about 10 minutes before slicing. Serve warm.