In a pressure cooker, bring the mirin and sake to a boil along with the ginger, shallots and garlic. Boil for 2 minutes to burn off the alcohol. Add the water and miso and stir to dissolve the miso. Add the mackerel, then seal and lock the lid. If your pressure cooker has settings set it for the highest pressure. Bring the pressure cooker up to pressure over medium high heat, then turn down the heat as low as you can while maintaining pressure and cook for 45 minutes. If you don't have a pressure cooker, you can simmer this in a pot with a lid for about 2 1/2 hours.
While the mackerel is cooking use a mandoline to slice the onion and celery very thin, then put them in a large bowl of ice water. Change the water 2 times every 20 minutes (for a total of a 1 hour soak). This will take the harsh edge off both the celery and the onion and it will also make the vegetables more crispy. Whisk the vinegar, mustard, sugar and salt in a small bowl to make the dressing.
When the mackerel is done, follow the pressure cooker directions to release the steam. The bones should be soft enough to eat. If they're not, you can add some more water if needed, reattach the lid and continue to cook another 10-15 minutes.
Drain the onions and celery and use a paper or a salad spinner to remove as much water as possible from them. Pour on the mustard dressing then toss to coat. To serve, just put down a bed of salad, and top with the braised mackerel and serve with a side of rice.