Step 1

Preheat oven to 375 degrees Fahrenheit.

Step 2

Place cod in a casserole dish (about 8-inch x 11-inch or something similar) and sprinkle with 1/4 teaspoon of salt. Set aside until you make the sauce. See "Notes" below if using cod fillets that have been previously frozen.

Step 3

In a small saucepan on the stovetop, heat milk over low heat just to warm through (the milk will be used shortly for the sauce).

Step 4

Meanwhile, in another saucepan on the stovetop, heat 1 tablespoon of olive oil over medium-low heat. Add onion and cook, stirring often, until softened, about 5 minutes.

Step 5

To onion, add butter. Once the butter melts, add flour and stir to form a paste. Cook, stirring often, about 3 to 5 minutes. Slowly add the warmed milk to this (1/2 cup at a time), whisking after each addition until thickened, about 3 to 4 minutes. Sauce mixture should coat the back of a spoon (mixture will be thick).

Step 6

To sauce mixture, add remaining 1 teaspoon of salt, black pepper, lemon zest and 1.5 teaspoons of dill (you can use less, if you prefer). Stir to combine. Pour sauce mixture over cod, coating all fish. Stir to ensure mixture is incorporated well.

Step 7

Sprinkle cheddar cheese and croissant crumbs (or regular bread crumbs) on top of sauced fish mixture. Bake in preheated oven for 40 minutes until bubbly and golden brown.

Step 8

Once done, sprinkle remaining 1/4 teaspoon of dried dill over top. Let cool for about 20 to 30 minutes before serving (don't skip this; it allows things to settle). Serve with lemon wedges (if desired). Enjoy!