Boil potatoes in salted water until very tender, about 15 minutes, and drain well.
Mash with 3 tablespoons butter, warm milk, half the salt and half the white pepper until smooth.
Melt remaining butter in a skillet, whisk in flour for 1 minute and gradually whisk in broth.
Add poultry seasoning and remaining salt and pepper and simmer until smooth and lightly thickened.
Lay turkey slices in the gravy, cover and warm gently for 5 minutes without boiling.
Serve turkey and gravy over rounded scoops of mashed potatoes.