Preheat the oven to 350 degrees F.

Place a large sheet of parchment paper on a baking sheet and place the cod fillet on the center.  Fold in the sides of the parchment paper to create a little boat (or crumble the sides just to ensure the melted butter and lemon juice doesn't escape.

In a small bowl, combine the melted butter (or avocado oil), garlic, lemon zest + juice and rosemary sprigs and stir well to combine. Drizzle this mixture over the cod fillet. Sprinkle the fish with sea salt and onion powder. Lay slices of lemon or orange over the fish.

Wrap the fillet into a parchment packet, using twine to secure either end. Bake for 25 to 35 minutes, or until fish is cooked through. Unwrap, and enjoy with choice of side dishes.