Grind lamb and beef together, using large plate on meat grinder.
Regrind using small plate.
Sweat off the onions.
In another small bowl combine next 6 ingredients and mix thoroughly.
In large bowl combine meat, onions and spice mix.
Add garlic and vegetable oil; mix thoroughly with hands until spices are evenly distributed; refrigerate, covered for several hours.
In a small skillet, fry a small amount of the meat mixture in a little butter.
Taste for seasoning and adjust, if desired.
Blanch bacon by simmering in hot water about 3 minutes.
If pork belly is used, blanching is not necessary.
Drain on paper towels.
Line loaf pan (8 1/2 x 5 x 3 inch) with bacon or pork slices, covering sides and bottom; pack gyros meat into pan, filling to the top of the pan.
Cover with remaining slices of bacon or pork.
Place filled pan in hot water bath, reaching 3/3/4 of the way up the loaf pan.
Bake at 375 degrees for 20-30 min.
Reduce heat and continue baking 30 min.
longer.
Remove pan from oven; remove loaf pan from hot water bath and place on wire rack to cool.
Cover with aluminum foil and weigh with about 2 lb weight to compress loaf.
Cool to room temperature; refrigerate meat, covered with weight 12 hours or overnight to ripen and increase flavor.
Meat will keep for up to 4 days refrigerated.