Heat the oven: Preheat to 425°F (220°C). Line a sheet pan with parchment or lightly oil a baking dish.
Prep the fish: Pat cod fillets dry with paper towels. Season both sides with salt and pepper.
Place on the prepared pan.
Make the herb crust: In a bowl, combine 1 cup panko, 2–3 tablespoons finely chopped parsley, 2 tablespoons chopped dill or chives, 1 grated garlic clove, and the zest of 1 lemon. Add 2–3 tablespoons olive oil (and 1 tablespoon melted butter if using) and a pinch of red pepper flakes. Season with a little salt and pepper.
The mixture should feel lightly moistened and hold together when pressed.
Add Dijon: Spread a thin layer of Dijon over the top of each fillet. This helps the crumbs stick and adds a subtle tang.
Top with crumbs: Spoon the herb mixture over the fish. Press gently so it adheres, covering the surface evenly.
Bake: Cook for 10–14 minutes, depending on thickness.
The crust should be golden and the fish should flake easily with a fork. For extra color, broil for 1 minute at the end, watching closely.
Rest and serve: Let the cod rest for 2 minutes. Squeeze fresh lemon over the top and serve immediately.