Using a mandoline, shave the fennel and radishes into paper-thin slices. If you don't have a mandoline, slice the veggies as thin as you can.
Put the lemon juice, dill, shallot, mustard, salt, sugar in a blender and pulse briefly to combine. With the motor running, gradually drizzle in the olive oil until the dressing emulsifies.
Toss the sliced fennel and radishes with the dressing and set aside to marinate while you're cooking the fish.
Pulse the buckwheat in a blender until well pulverized. Then toast the buckwheat powder on a dry skillet, until lightly golden and aromatic, for about 4 min. Remove from the pan and set aside.
In a small bowl, combine the fish sauce, lemon juice, garlic, and chili flakes. Set the flavored fish sauce aside until needed.
Heat a small amount of olive oil in a large non-stick pan over medium-high heat. Lightly sprinkle the salmon with salt, then dredge the skin of the salmon in buckwheat powder. Fry the fish skin side down until crispy and golden brown, then flip on the other side and cook for just a couple of minutes until medium-rare. Remove the salmon from the pan and let rest while you're finishing the sauce.
In a small saucepan, gently brown butter until golden and aromatic. Add the orange segments, chopped walnuts, and as much flavored fish sauce as you want. Mix with a small whisk to emulsify, then add chopped herbs and remove from the heat.
Serve the salmon with the brown butter sauce drizzled on the plate and a generous amount of fennel-radish slaw.