Preheat the oven to 400°F. Prepare any additional side dishes that you have planned before working on the fish.
Lay out 3 pieces of foil, approximately 14-18" in length. Top each piece of foil with a piece of parchment paper, also 14-18" in length.
Mist each piece of parchment with the cooking oil spray.
Place the zucchini slices in a 3-slice by 4-slice rectangle in the middle of each sheet of parchment paper. Put any additional zucchini slices that are left on top of the rectangles. (If you use extremely large or small zucchini, the number of slices used per cod packet will vary.)
Put a cod fillet on top of each zucchini rectangle.
Pour ⅓ of the lemon juice on top of each piece of cod. Sprinkle the lemon juice, lemon zest, garlic, salt, pepper, sage, and thinly sliced onions on top (in that order).
Put 2 pats of butter on top of the food in each packet (i.e., each packet gets one tablespoon of butter total).
Fold the parchment snugly around the food in each packet. You want the food to be completely covered in parchment. I folded the sides of each packet and then the top and bottom. Secure the parchment by folding the foil over the parchment in a similar way. First fold in the sides of the foil, and then the top and bottom. Use the recipe video below as a guide in folding your parchment packets.
After each packet of food has been completely encased in the parchment and then the foil, you are ready to get steaming. Place the packets in a single layer on a large baking sheet.
Put the baking sheet with the packets in the top third of your preheated oven. Steam for 18-20 minutes at 400°F.
After the allotted cooking time, unwrap your packets. The fish should be tender and flaky.
To serve, plate the contents of the fish packets with any additional side dishes that you have prepared. I like to serve the fish and lemon-butter sauce in the packet on top of a cooked whole grain. Sometimes I serve the red onions on top of the fish, and sometimes I mix them with the cooked zucchini on the side.