In a large bowl, combine the ground meat, finely chopped mushrooms, minced garlic, and any desired seasonings like salt and pepper. Mix thoroughly.

Gently separate leaves from Napa cabbage. Place a tablespoon of filling at the base of each leaf, fold in sides, and roll tightly. Secure with a toothpick if necessary.

In a medium saucepan, whisk together soy sauce, oyster sauce, mirin, and sugar over medium heat until well combined and simmering.

Set up a steamer basket over boiling water and arrange cabbage rolls in a single layer. Steam for 10 to 12 minutes until tender.

Prepare cornstarch slurry and stir into simmering sauce. Continuously stir for another 2-3 minutes until thickened.

Transfer steamed rolls to a serving platter and drizzle with thickened sauce. Garnish with chopped scallions.