Prepare soymilk broth by mixing together all ingredients in a bowl. Adjust to taste and set aside.
Slice napa cabbage into quarters, wash and set aside. Slice kabocha squash into quarters, remove the seeds using a spoon to scoop and then thinly slice the kabocha squash using a mandolin. You want to aim for about 1/8" thickness, or thin enough to be pliable. Layer kabocha squash slices in between each leaf of cabbage. Repeat with remaining quarter of cabbage. When ready to assemble, slice the wedge into 3 parts, horizontally.
You should end up with alternating cabbage and kabocha squash layers. Add these to the pot you're using and fill the perimeter. Add the shimeji mushrooms into the center of the pot to secure the cabbage layers.
If desired, thinly slice leek on the diagonal and add some slices in between the kabocha and cabbage layers. Cut carrots thinly and shape like flowers if desired, then place on top of the cabbage leaves.
Pour the broth into the pot, bring to a boil then lower to a simmer and let cook for 10-15 minutes, until ingredients are all tender. If needed, skim any foam that comes to the surface while simmering.