Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
Pat the cod fillets completely dry with paper towels. Season both sides lightly with salt and pepper and arrange them on the prepared baking sheet.
In a small bowl, mix together the mayonnaise, Dijon mustard, lemon juice, and minced garlic.
In a separate bowl, combine the panko breadcrumbs, Parmesan cheese, parsley, paprika, 1/2 tsp salt, and 1/4 tsp pepper. Drizzle with the melted butter and mix with a fork until evenly moistened.
Spread a thin layer of the mayo mixture over the top and sides of each cod fillet.
Press the panko mixture firmly onto the mayo-coated fillets, mounding it generously.
Bake for 15-20 minutes, or until the topping is golden brown and the fish flakes easily with a fork.
Let rest for a minute before serving, with an extra squeeze of fresh lemon juice if desired.
Notes
The most crucial step for a crispy topping is to pat the cod fillets completely dry with paper towels before seasoning. Any excess moisture will turn to steam and make the crust soggy.