Prepare the Oven and Pan: Preheat the oven to 425°F (220°C). Line a large rimmed baking sheet with parchment paper or aluminum foil. This step ensures that the salmon does not stick to the pan and makes for easier cleanup after baking.

Dry and Season the Salmon: 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. Drying the salmon helps the crust adhere better and prevents it from sliding off during baking.

Make the Parmesan Topping: 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. This texture is crucial for achieving a crispy topping.

Add the Mustard Layer: 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. The mustard acts as a flavorful adhesive for the topping.

Apply the Crust: 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. A well-applied crust will enhance both flavor and texture.

Bake the Salmon: 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. This step is key to ensuring the salmon is cooked through while keeping it moist.

Check the Internal Temperature: 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. Monitoring the temperature is the best way to ensure perfect doneness.

Brown the Topping if Needed: 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. This step adds an appealing golden color and enhances the crust's texture.

Rest the Salmon: 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. Resting allows the juices to redistribute, ensuring a tender bite.

Garnish and Serve: Transfer the fillets carefully with a thin spatula. Garnish with the remaining parsley and serve with lemon wedges, if using. This final touch adds freshness and a pop of color to the dish.