Peel and shred carrots. Cook carrots with canola oil until carrots turn dark orange (see picture).
Blanch napa cabbage leaves in salt water, remove and drain. Score the outer base of the leaves, turn over, tuck and roll each leaf into a tight roll; pin in place with a toothpick and place on steam tray. Steam the rolls until tender (about 10 minutes). Remove toothpicks and place rolls on serving platter.
Add water, salt, mushroom seasoning, and cooked carrots. Add potato starch premixed in water to thicken. Pour sauce over the Napa cabbage rolls and serve.