Combine the lamb, salt, pepper, and oregano in a medium bowl. Mix with hands until well blended then cover and refrigerate at minimum one hour or up to over night.
Process onion in a food processor to get a fine chop, about 10 - 15 seconds. Then turn out onto a tea towel and wring out onion liquid. Reserve.
Preheat oven to 300 degrees. Line a shallow rimmed baking sheet with foil and reserve.
Place cold lamb mixture in the bowl of a food processor, with onion, garlic, and bacon. Process until a fine paste-like mixture is formed, scrapping down edges as you go, about 1 minute total.
Form mixture into a loaf about 1 1/2" high, 8" long, and 5" wide on the foil lined baking sheet. Bake for 35 - 40 minutes until internal temperature reaches 155 degrees. Remove from oven and allow to cool to room temperature, about 15 minutes.
Adjust oven rack to highest broiler position and preheat the broiler. Slice loaf crosswise into 1/8th to 1/4-inch thick slices. Lay slices on foil lined baking sheet and broil until edges are brown and crispy, about 2-3 minutes. Remove and tent with foil. Warm bread under broiler, turning after 45 seconds per side.
To serve...
Spread Tzatziki Sauce on the center of each pita. Layer with sliced tomato, lettuce leaves, red onion, and gyro meat slices. Top with another dollop of sauce.
