Put mayonnaise, grated Parmesan cheese, Dijon mustard, and minced garlic into a mixing bowl. Add chopped flat leaf parsley, juice of ½ lemon or 1½ Tbsp lemon juice, and 1 Tbsp lemon zest. Season with salt and pepper. Mix all the ingredients.
Place the cod fillets in a 9x13 inch baking dish or on a baking sheet. Spread the mixture generously on top of the fillets.
Sprinkle the Panko breadcrumbs on top and gently press them into the mixture.
Bake in a 400°F preheated oven for 20 minutes until the fish is cooked through and the crust is golden brown. Garnish with extra lemon zest and chopped parsley if desired. Serve warm.