1. Preheat and Prepare: Start by heating your oven to 400°F (200°C). Line a rimmed baking sheet with parchment paper to ensure the delicate skin doesn't stick and cleanup is a breeze.
2. Mix the Crust: In a small glass bowl, combine the breadcrumbs, parsley, dill, lemon zest, and olive oil. Use your fingers or a small fork to massage the oil into the herbs until the mixture looks like coarse, damp sand. The smell of the lemon hitting the fresh herbs is exactly what we want to capture.
3. Prime the Salmon: Pat your salmon fillets dry with a paper towel. This is crucial—removing surface moisture helps the mustard adhere and prevents the fish from "steaming" in the oven. Lightly season the tops with salt and pepper, then brush a thin, even layer of Dijon mustard across each fillet.
4. The Crust: Gently press the herb mixture into the mustard layer. Don't be afraid to give it a firm tap so the herbs stay put during the bake.
5. Roast: Slide the tray into the center rack of your oven. Bake for 12–15 minutes. You are looking for that golden-brown color on the crust and a light, opaque flake when you nudge the center of the salmon with a fork.

Leftover salmon is rarely as good as fresh, but you can minimize the quality loss. Store cooked fillets in an airtight glass container in the refrigerator for up to 48 hours. To reheat, I recommend using a toaster oven or a standard oven at 300°F for about 8 minutes. Avoid the microwave if possible, as it will turn the protein rubbery and ruin the crunch of your herb crust.