Prepare sauce by mixing all ingredients in a small bowl, stirring to blend well. Taste to adjust salt and pepper if needed. Set aside.
Mix beef and remaining ingredients except lettuce leaves in a medium bowl. Press into bowl and use a knife to divide the mixture evenly into 8 parts (divide mixture in half first, then in half again, and so on to make the patties similar sizes).
Using one of the portions, shape into a small oval patty and set on a plate or tray. Repeat with remaining meat portions. The patties can be made in advance and kept in the refrigerator until needed.
Heat a grill to medium-high and cook patties until done, about 3-4 minutes each side or until an instant read thermometer reaches above 150 degrees (or to your desired doneness).
Place each patty in a lettuce leaf and drizzle with yogurt garlic sauce. Serve with remaining sauce.
Serving Idea:We like to serve this with hummus and cut vegetables - and if we were really in Greece, a plate of French fries (really- they served them with everything!)