Preheat oven to 425 degrees Fahrenheit. Line a baking sheet with parchment paper.
Add salmon fillets to prepared baking sheet, about 2 inches apart and skin side down.
In a bowl, combine dijon mustard, melted butter, salt, and pepper. Stir to combine. Spoon mustard sauce over fillets and spread into an even layer. Set aside.
In a skillet over medium heat, add panko bread crumbs and toast until golden brown, stirring frequently. This should take about 3-4 minutes. Remove from heat and add lemon zest, stirring to combine.
Sprinkle bread crumbs over salmon, pressing gently so that bread crumb layer sticks to dijon layer.
Bake for 10-12 minutes (for a 1 inch thick salmon fillet), or just until salmon flakes apart easily in the center when tested with a fork.
Before cooking, check your salmon fillet for pin bones. Gently run your hands across the top of your salmon fillet. You'll feel a hard bit protruding from your fillet. Use pliers to gently pull the bone out.
Testing for doneness - use a fork to pierce into the largest part of your salmon fillet. If it flakes apart easily, it's done. Or, use a thin metal rod like a cake tester or skewer to poke into the center of your salmon. Then, touch the skewer with the back of your hand. If your salmon is hot in the center, it's done. If your salmon is barely warm or cold, it needs more time in the oven.