Pour gravy into an ice cube tray and freeze until the cubes are solid and can be removed easily.
2. Step
Place the potatoes into a pot with cold, salted water and bring to a boil and cook until the potatoes are fork-tender.
3. Step
Drain the potatoes until completely dry.
4. Step
While still hot, mash the potatoes with a potato masher, fork, potato ricer, or food mill. If the potatoes get too cold, reheat them in the oven or microwave until hot and dry again.
5. Step
To the mashed potatoes, add the sour cream, butter, egg yolks, salt, pepper, and nutmeg. Fold the mixture together with a spatula until just incorporated, making sure not to overmix.
6. Step
Transfer the mixture to a piping bag or a zip-top bag with a star tip.
7. Step
Pipe a disc of the mashed potatoes on a parchment paper-lined sheet tray.
8. Step
Place an ice cube of frozen gravy in the center, and pipe more mashed potatoes around the gravy cube,
9. Step
Freeze until the potatoes are firm.
10. Step
Preheat oven to 425°F (220°C).
11. Step
Brush with egg wash until completely coated.
12. Step
Bake for 30 minutes, or until the potatoes are heated all the way through and golden brown.
