Heat oven to 425°F. Line a sheet pan with foil and lightly oil it.
Pat salmon dry and set skin-side down on the pan. Season lightly with a pinch of the salt and pepper.
Stir Dijon and mayonnaise together, then spread the mixture evenly over the top of the salmon.
Combine panko, Parmesan, parsley, garlic, lemon zest, olive oil, melted butter, remaining salt, pepper, and red pepper flakes. Toss until crumbs are evenly moistened.
Press the crumb mixture over the salmon to coat the top in an even layer.
Bake until the crust is golden and the salmon flakes easily, about 12–14 minutes depending on thickness.
Rest 2 minutes, then drizzle with lemon juice and serve warm.