Cut the bok choy into stems and leaves. Slice the stems into quarters lengthwise and the leaves into 3cm wide pieces. Slice the lotus root into thin half-moon shapes and soak in water to remove excess starch. Remove the pit from the ume plum and mash it with a knife.

Cut the pork belly into 3cm wide strips.

Layer bok choy, lotus root, and pork belly in a heatproof container, repeating until all ingredients are used. Sprinkle with salt and pour sake over the layers. Cover loosely with plastic wrap and microwave at 600W for 7 minutes. Let sit for 2 minutes to finish cooking with residual heat. Reserve 1 tbsp of steaming juice.

Mix the reserved steaming juice with the ume plum, sugar, soy sauce, and sesame oil in a bowl to create the ume sauce.