Baked Mashed Potatoes

Preheat the oven to 350 degrees F (175 degrees C). Grease a 2-quart casserole dish.

Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce the heat to medium-low and simmer until tender, about 20 minutes.

Drain potatoes and transfer to a large bowl. Add butter and milk, and mash to combine. Add cream cheese and onion; beat with an electric mixer until smooth.

Whisk egg in a small bowl. Add one spoonful hot potatoes, whisking quickly to avoid scrambling the egg. Pour mixture back into the bowl and stir to combine with remaining potatoes. Season with salt and pepper. Transfer to the prepared casserole dish.

Bake in the preheated oven until puffy and golden brown, about 1 hour.