I made a variation of this over the weekend and it was so good I wanted to try again. Cooking in a foil pouch allows you to do this equally well on the grill (as I did) or in an oven. Cod is a great fish for this method as it handles the heat very well and if you happen to over cook it a bit it still remains quite moist.
You will need
One large cod filet, about an inch to 1.5 inches thick
Half a bunch of swiss chard
4-5 cloves of garlic
1 small onion
2 stems of tarragon ( I used rosemary the first time it works great)
Lay out a double layer of foil. Slice the onion and garlic and lay them down on the foil in a pile. Add your sprigs of fresh herb. Roughly chop the swiss chard and mound it up in a pile about as long as your fish. A couple inches tall should be perfect. Drizzle with olive oil and sprinkle on kosher salt and pepper. Lay the fish on the top of the pile, sprinkle on some salt and pepper and a bit more olive oil. Seal the foil tightly and cook on a medium grill or in the oven at 400 for about 10-12 minutes.
The fish should be just starting to flak and the chard should be fully wilted.
I really like the Tarragon this time around as it perfumed and lightly flavored the fish with its delicious licoricey goodness. However if you are really averse to it then go ahead and use some rosemary in its place.