Mince the onion and place it in a microwave-safe container. Cover loosely with plastic wrap and microwave at 600W for 1 minute and 30 seconds. Drain excess water and let it cool.

In a bowl, soak breadcrumbs in milk. Add ground meat, onion, salt, black pepper, and nutmeg, then mix well. Divide into 6 portions.

Spread out cooking paper, place the meat mixture on it, and roll it up tightly. Twist both ends to secure. Arrange in a microwave-safe container and microwave at 600W for 2 minutes. Flip and microwave for another 2 minutes. Let cool.

In a bowl, mix grain mustard and Worcestershire sauce to create the mustard sauce.