Peel and dice the Russet potatoes, then place them in cold salted water and bring to a boil. Cook for 15-20 minutes until tender.

Mash the warm potatoes until smooth, then mix in cheddar cheese, sour cream, and softened butter until creamy.

In a skillet, heat butter over medium heat, add onion, and sauté for 3-4 minutes until translucent. Stir in garlic and thyme, cooking for another minute.

Fold the sautéed onion mixture into the creamy potatoes, season with salt and pepper to taste.

Preheat the oven to 375°F (190°C), roll out the pie crust, fill with potato mixture, and fold edges if desired.

Brush the crust with a beaten egg and bake for 30-35 minutes until golden brown and bubbling.

Let the pie cool for 10 minutes before serving to allow the filling to set.