Process the onion to small chop. Remove from processor and press out the excess water. Return the onion and add all ingredients to the processor and pulse to a paste, scraping sides as needed.
Press meat mixture into a loaf pan. Place loaf pan in a water bath. I used a 13 x 9 pan with about 1 inch of water.
Bake for about an hour or until internal temp reaches 165.
Remove from oven and drain off fat. (Alton Brown instructs to place a foil covered brick on top of the loaf. I didn't have one so I skipped this step) Allow to rest on a wire rack for 15 to 20 minutes or until meat is a manageable temperature for slicing.
Fill pita bread with gyro meat, tomato, onion, kalamata olives, or your favorite Greek toppings, and crumbled Feta cheese. Finish with Tzatziki Sauce and serve.
I've prepared this recipe a bunch of times and have yet to try the brick weight. I would imagine the weight would improve the density of the finished loaf.
On my first attempt, the Gyro meat was a bit crumbly upon slicing. Second time I processed a little longer and found that to eliminate the crumbling problem.
Home made tzatziki sauce is always a winner here but in a pinch one could substitute with ready made tzatziki or cucumber ranch dressing mixed with a bit of greek yogurt.