Add remaining 1 tbsp butter and decrease heat. Add flour and whisk until fully incorporated. Slowly add warm milk while whisking constantly. As the sauce starts to thicken add lemon zest, mustard, salt, pepper and herbs (if using) and whisk until well blended. Add half the parmesan cheese onto the sauce and turn off heat.
Grease an oven-safe dish and lay the fillets on the bottom.
Pour sauce over the cod fillets.
In a bowl mix croutons with olive oil and mash them up a bit.* Sprinkle mashed croutons over the dish and top with remaining parmesan cheese.
Bake at 350 for 30-40 minutes.
Notes
*I used old stale bread, cut it in little cubes and threw them in the food processor with 2 tsp olive oil to make a mix of breadcrumbs with a couple of big pieces.
Recipe by Enjoy Tribute at https://enjoytribute.com/newfie-cod-au-gratin/