Julienne the potatoes and soak them in water to remove excess starch. Thinly slice the onion and cut the ham into thin strips.

Place the potatoes and onions in a microwave-safe container, cover loosely with plastic wrap, and microwave at 600W for 4 minutes until the potatoes are tender.

In another microwave-safe container, mix the heavy cream, grated cheese, salt, black pepper, and egg yolk. Cover loosely with plastic wrap and microwave at 600W for 1 minute.