Combine the wasabi powder, rice vinegar and water in a small bowl. Let rest for 1 hour.
In a large stockpot, place the potatoes and a generous pinch of salt. Cover with cold water and bring to a simmer. Cook at a simmer until the potatoes are fork-tender, about 15 minutes. Drain the potatoes and return to the pot. Add the butter, milk, buttermilk, salt and pepper. Mash until smooth. Fold in the wasabi paste.
In a medium saucepan, heat the soy sauce, ½ cup water, sugar, scallions, garlic, sake, dried shiitakes, and ginger. When the mixture reaches a boil, use a strainer to remove the vegetables. Simmer for 20 minutes. In a small bowl, combine the cornstarch with 1 tablespoon water to form a paste. Whisk into the teriyaki sauce until thickened. Remove from the heat.
Meanwhile, bring a large pot of salted water to a boil for the broccolini. Prepare an ice bath by filling a large bowl with water and ice. When the water boils, add the broccolini for 2-4 minutes, until bright green and crisp-tender, then immediately transfer to the ice bath to stop the cooking process. Remove from the ice bath after 1-2 minutes.
Place the miso, mustard, lemon juice, soy sauce, and water into a blender. Purée until smooth. With the motor running, drizzle in the oil until emulsified. Season with salt and pepper. Toss the broccoli with the miso vinaigrette.
For the tuna, combine the cilantro, garlic, soy sauce, lemon juice, sesame oil, brown sugar, ginger and sambal in a blender. Purée until smooth, adding water if necessary. With the motor running, drizzle in the oil until emulsified. Season with salt and pepper. Coat the tuna with the tamari vinaigrette.
Heat a grill or large skillet to high heat and sear each side of the tuna for 1-2 minutes, until browned. Slice on the diagonal.
To serve, divide the mashed potatoes, salmon steaks and miso broccolini between plates. Top with teriyaki sauce.