Start by rubbing the duck legs with 1 slice of ginger. Next, pour 2 tablespoons of mirin over the duck and rub it in. Let this stand for 15 minutes. Doing so will remove the gaminess and any underlying odors.
In the meantime, char the onion on the stovetop until most of it is blackened. You can also broil the onion.
To a large pot with boiling water, add the duck legs, grilled onion, ginger, 2 tablespoons of mirin, and salt.
Bring to a boil then skim off the scum on the top. Cover and simmer on medium-low for 45 minutes to 1 hour.
Remove the legs and immediately plunge them into an ice bath. Scoop out the aromatics then reserve the broth and repurpose it however you’d like. I use the broth to make Duck Congee! (see note 1)
Once the duck has cooled, chop it into bite-sized chunks. Then set aside for later.
Using a mandolin, carefully shred the cabbage and thinly slice the onions.
Lay the cabbage on a plate then add the onions and cilantro on top. Pour half the sauce over the vegetables and aromatics.
Then arrange the sliced duck on top of the plate. Garnish with more cilantro and fried shallots.
Serve the remaining sauce on the side as a dipping sauce for the duck.