Cook potatoes, onion, and celery in a pot with minimal water until tender.

Drain the potatoes, reserving the cooking water.

Mash the cooked potatoes, onion, and celery together with the reserved cooking water.

Add butter, egg, milk, salt, pepper, sage, and poultry seasoning to the mashed potato mixture.

Stir in parsley flakes.

Gradually add bread crumbs or toasted bread to the mixture until the desired thickness is achieved.

Butter a casserole dish.

Pour the mashed potato filling into the buttered casserole dish.

Bake in a preheated oven at 350°F (175°C) for 45 minutes, or until golden brown.