Preheat the oven to 425°F (220°C). Line a large rimmed baking sheet with parchment paper or aluminum foil.

Pat the salmon fillets thoroughly dry with paper towels. Place them on the prepared baking sheet, leaving space between each piece. Season the tops and sides evenly with the salt and black pepper.

In a small bowl, combine the panko breadcrumbs, Parmesan cheese, 2 tablespoons parsley, lemon zest, and garlic powder. Pour in the melted butter and stir until the crumbs are evenly moistened. The mixture should resemble loose, damp sand rather than a wet paste.

Spread approximately 1 1/2 teaspoons of Dijon mustard over the top of each salmon fillet. Keep most of the mustard on the upper surface so the crust does not slide down the sides.

Divide the Parmesan mixture among the four fillets: Press it gently into the mustard layer without compacting it heavily. The topping should form an even layer that remains loose enough to crisp.

Bake on the center rack for 10 to 15 minutes, depending on the thickness of the fillets. The topping should be golden, and the salmon should begin to flake when gently pressed with a fork.

Insert an instant-read thermometer into the thickest part of a fillet, avoiding the baking sheet. The salmon should reach 145°F (63°C). Remove thinner pieces as soon as they are done so they do not dry out.

If the salmon is cooked but the crust remains pale, switch the oven to broil and cook for 30 to 60 seconds. Watch continuously because the Parmesan and breadcrumbs can darken quickly.

Remove the baking sheet from the oven and let the salmon rest for 5 minutes. The fish should remain moist and separate into broad flakes when pressed.

Transfer the fillets carefully with a thin spatula: Garnish with the remaining parsley and serve with lemon wedges, if using.