Preheat the oven to 425 degrees F and coat a baking sheet with oil and sprinkle it with salt and pepper.
Place the potatoes, cut side down, on the tray and place in the preheated oven. Roast until just golden brown and just tender, about 20 to 25 minutes. Remove the potatoes from the oven and hold them in a warm place.
Mix the horseradish, parsley, lemon zest, mustard, and 2 tablespoons of garlic in a small bowl. When it is evenly combined, add the bread crumbs and toss them gently. Drizzle 3 ounces of the olive oil over the crumb mixture and stir gently taking care not to break up the crumbs or make the mixture too heavy. Season the mixture, to taste, with salt and pepper.
Season the salmon fillets with salt and pepper then heat the canola oil in a saute pan over medium high heat. When the oil just begins to smoke, sear the salmon paves, topside down, 2 pieces at a time, for 2 to 3 minutes. Remove to a lightly greased baking tin, placing the seared side up and repeat the process with the other 2 pieces of salmon. Sprinkle the bread crumb mixture over the top of the fish creating an even layer 1/8-inch to 1/4-inch thick. Roast the salmon until the crust is golden brown and fish is just firm, approximately 5 to 7 minutes.
While the salmon is baking, bring the fish stock, remaining 1 ounce olive oil, and remaining tablespoon of garlic to a boil in a non-reactive saucepan. Then add the braising greens, cover and cook for 2 to 3 minutes until the greens are wilted. Add the lemon juice and season, to taste, with salt and pepper. Arrange the greens in the center of 4 shallow bowls surrounded by the roasted potatoes and braising liquid. Top each with a roasted pave of salmon.