Heat the oven to 400°F and grease a 9-by-13-inch baking dish.
Boil the penne for 3 minutes less than the package directs, add the spinach for the last 30 seconds, then drain both well.
Melt the butter in the pasta pot, whisk in the flour for 1 minute and gradually whisk in the milk until smooth.
Simmer the sauce for 3 to 4 minutes until thickened, then stir in the lemon zest, 3/4 cup Parmesan, salt and pepper.
Fold the pasta, spinach and cod gently into the sauce and spread the mixture in the baking dish.
Mix the panko, olive oil and remaining Parmesan and scatter it over the casserole.
Bake for 18 to 22 minutes, until the cod flakes easily, reaches 145°F and the crumbs are golden.
Rest for 5 minutes before serving.