Step 1

Boil potatoes until soft in salted water

Step 2

In the bowl of a standing mixer, add potatoes, salt and pepper to taste, and 4 tablespoons butter, then gradually beat until smooth

Step 3

In a pan, heat 2 tablespoons butter over medium heat

Step 4

Add the onion, celery, and parsley and cook until celery softens and onions are clear but not brown

Step 5

Add the parsley mixture to the whipped potatoes, then fold in the egg

Step 6

Soak the bread in the milk

Step 7

When the bread has absorbed most of the milk, fold the bread mixture into the potato mixture

Step 8

The consistency should be slightly runnier than very soft mashed potatoes; if not, add a touch more milk

Step 9

Scrape into a buttered casserole or soufflé dish and top with a pat of butter

Step 10

Bake in a preheated 350˚F oven until crusty and golden brown, about 1 hour