Put the walnuts in a food processor and blend until you have a nice almond meal. It doesn't have to be too finely ground, just evenly ground. You actually don't want to overdo it with the processor or you might end up with walnut butter instead!
Add in the onion powder and lemon zest and pulse several times with the processor to mix them in.
Place the fish on a plate and pack on the walnut topping. It will become slightly moist and sticky but it will still be workable. You do not have to cover the fillet entirely, and do not coat both sides of the fish (it won't bake properly). This is meant to be a topping, not a breading.
When the top of the cod is covered, transfer to a baking dish.
Bake at 350 F. for about 20-25 minutes or until the fish is fully cooked. Baking time will vary a bit by oven.
Then turn your oven to broil and continue cooking for another 5-10 minutes, or until the walnuts are a golden brown from roasting.
Transfer to plate, allow to cool slightly.
Serve with some fresh lemon juice (wedges) and some fresh, chopped parsley for garnish.
Season with salt as needed.
Please note that the nutrition data is a ballpark figure. Exact data is not possible.