Cod au Gratin

1 1/2 pounds cod (about 3 large fillets, cut into bite sized pieces)
2 cups milk
1 tablespoon olive oil
1 onion (minced)
4 tablespoons unsalted butter (1/4 cup)
4 tablespoons all purpose flour
1 1/4 teaspoons salt (divided)
1/2 teaspoon black pepper
1 tablespoon lemon zest (reserve lemon for wedges, 4 later)
1 3/4 teaspoons dried dill (divided)
2 cups shredded cheddar cheese
1 croissant (made into bread crumbs or 1 cup bread crumbs)

If using cod that has been previously frozen, pre-cook in a casserole dish and lay to one side
Cook onion on the stove top until softened.
In a pot, make roux (flour/butter mixture to thicken sauce).
Add milk to roux, cook until thickened a little, and season with salt, pepper, lemon zest and dill.
Pour sauce mixture over cod (layer), top with cheese and bread crumbs.
Bake until bubbly and brown.