Place oven rack in upper third of the oven and preheat to 425°F.
Prepare a rimmed baking sheet by lining it with parchment and spraying it with cooking spray.
Place the salmon skin side down onto the prepared baking sheet. Remove any pin bones from the fillets, if necessary. With a small rubber spatula or offset spatula, evenly spread the 6 tablespoons of mustard over the entire fillet(s).
In a mixing bowl, combine the 1/3 cup chopped pecans, 1/3 cup Panko bread crumbs, 1/4 teaspoon kosher salt, and 1/8 teaspoon pepper. Mix well and cover the salmon.
Bake for 10-12 minutes. Turn the broiler on to high and cook for 1-2 minutes, just until tops are golden. Watch carefully as it can burn very quickly.
Notes
Choose a center cut fillet(s) as the thinner tails will cook more quickly.