Preheat the oven to 350˚. Place the walnuts on a sheet pan and cook until lightly toasted, about 8 minutes. Transfer the walnuts to a cutting board and when cool enough to handle, roughly chop and set aside.
Increase the oven temperature to 400˚. Line a sheet pan with foil. Using paper towels, pat the fish fillets dry. Evenly coat each salmon fillet with 1/2 tablespoon of the mustard. Season the fillets on all sides with salt and pepper and then sprinkle 1/2 tablespoon of the flaxseed on top of each piece of fish, gently pressing them into the flesh. Place the fish on the lined sheet pan and drizzle each fillet with 1 tablespoon of the olive oil. Transfer to the oven and bake until the fish reaches an internal temperature of 125˚, about 10 minutes.
Meanwhile, in a medium bowl, whisk together the remaining 4 tablespoons olive oil, the vinegar and honey. Season with a pinch of salt and a twist of black pepper. Add the walnuts, grapes, parsley and scallions and toss together.
When the fish is done, remove it from the oven, top each piece with a generous amount of salsa and serve.