Pat the salmon dry using paper towels. This step is very important to get the skin crisp.
Rub all surfaces of the salmon with a bit of olive oil then sprinkle with the onion powder, salt, and pepper on both sides.
Use a microplane to grate the dried shiitake onto both sides of the salmon, forming a crust on the outside.
Heat a frying pan over medium-high heat until hot, add a splash of olive oil, and then place the salmon in the pan skin-side down. Cook until the skin is brown and crisp and the salmon has cooked about ⅓ of the way up each fillet. Flip them over and cook the other side until lightly browned then transfer to a plate to rest.
Make the dressing by whisking the olive oil, Meyer lemon juice, dijon mustard, salt and pepper together in a bowl until it's emulsified.
For the salad, put the mizuna leaves in a bowl with the julienned sunchokes, then drizzle with some of the dressing and toss to coat. Adjust the amount of dressing to taste. Toss to coat evenly.
Plate by putting down a bed of salad on a dish and topping it with the salmon. Serve with some grapefruit segments for a citrusy zing.