Mix all the sauce ingredients together and set aside. (It will keep in the fridge for 2–3 days.)
For best results, chop the scallion as finely as possible, almost like mashing it together with the umeboshi.

Fill a pot with water and a pinch of salt, and bring to a boil. Slice the carrot very thinly (a vegetable peeler works well), and tear the lettuce into bite-sized pieces.

Start with the lettuce. Set a bowl of ice water nearby. Add a handful of lettuce to the boiling water for about 2 seconds, then immediately transfer to the ice water to keep the color vibrant.

Next, cook the carrot slices to your preferred tenderness. Drain the lettuce and carrot in a colander to remove excess water, but do not squeeze them.

Now for the pork. Prepare a large plate lined with two layers of paper towels. Add a splash of sake (optional, not included in ingredient list) to the boiling water.

When the water is just below boiling (about 176°F/80°C), swish each slice of pork in the water until the color changes, then place it directly onto the prepared plate to cool.

Tip: For chilled pork shabu-shabu, avoid boiling water or cooling the pork in ice water, as this can make the meat dry and tough.

If using beef instead, remove it from the water as soon as the color changes and immediately cool it in ice water. This prevents the meat from becoming tough from residual heat, which is the opposite of pork.

Arrange the vegetables on a plate, top with the pork, and generously pour the scallion ume plum sauce over everything. Add tomatoes or other colorful veggies for garnish if you like.