Place the head of cabbage in a large microwave-safe bowl, and microwave for about 4 minutes, until the outer leaves begin to loosen.
Once cool enough to handle, remove the cabbage from the bowl. Gently remove 2 whole outer leaves from the cabbage, and place them back in the bowl. (Reserve the rest of the cabbage for another use.)
Add 2 tablespoons of water to the bowl with the cabbage leaves. Cover and microwave for about 2 minutes, until leaves are very soft.
Meanwhile, spray a skillet with nonstick spray and bring to medium heat on the stove. Add the burger patty and onion slice and cook for about 4 minutes per side, until cooked through.
Top one cabbage leaf with the burger patty, followed by a tomato slice, the onion, and the remaining tomato slice. If you like, top with ketchup and mustard. Gently top with the other leaf. Serve with pickles, if you want, and chew!