Place cabbage leaves in a microwave-safe container, cover loosely with plastic wrap, and microwave at 600W for 2 minutes. Let cool. Remove pits from umeboshi and mash them with a knife.

Finely chop shiso leaves and mix them with the sauce ingredients (sesame oil, soy sauce, black pepper) in a bowl.

Flatten the chicken thigh by cutting to even thickness. Poke holes with a fork on both sides and season with salt and black pepper.

Spread mashed umeboshi evenly on one side of the chicken. Layer cabbage leaves on top, roll tightly from one end, and secure with toothpicks.

Place the roll in a microwave-safe container, leaving space between rolls. Cover loosely with plastic wrap and microwave at 600W for 4 minutes. Flip the roll, cover again, and microwave for another 4 minutes. Let cool and remove toothpicks.