Place cabbage leaves in a microwave-safe container, sprinkle with 1 tbsp water, cover loosely with plastic wrap, and microwave at 600W for 4 minutes. Let cool, pat dry with paper towels, and trim the thick stem parts.

Slice the onion thinly and trim the root of the shimeji mushrooms, separating them into bite-sized pieces.

Place a cabbage leaf on a cutting board, layer with 2 slices of pork, season with salt and pepper, fold the edges of the cabbage, and roll tightly. Secure with a toothpick. Repeat to make 4 rolls.

Heat salad oil in a pot over medium heat, sauté the onion until softened. Add all tomato sauce ingredients and bring to a boil.

Place the cabbage rolls seam-side down into the pot, add shimeji mushrooms, and simmer covered on low heat for 15 minutes.