Place the onion in a food processor and pulse until finely chopped. Add the ground lamb and beef, the seasoning blend, 1 teaspoon salt, 1 teaspoon black pepper, and 2/3 of the grated garlic. Process until evenly combined and smooth.
Transfer the mixture to a 7x4-inch loaf pan. Place a piece of plastic wrap on top and press down firmly. Discard the plastic wrap and give the pan a few taps on the counter to make sure the mixture is compact as possible. Cover the pan tightly with foil and chill for at least 30 minutes or up to 3 hours.
In the meantime, in a medium bowl, stir together the yogurt, cucumber, parsley, dill, lemon juice, remaining garlic, salt and pepper until evenly combined. Cover with plastic wrap and chill until ready to use.
Place the loaf pan into a larger baking dish and then pour in hot water about half way up the loaf up, to create a steam bath. Bake in a preheated 325 degree F oven for about 1 hour. Carefully remove from the oven and drain the hot water. Unwrap the foil from the loaf pan and drain off any accumulated grease. Allow the meatloaf to cool for about 10 minutes before transferring to a cutting board and cutting into thin slices.
Place a large skillet over medium-high heat with the olive oil. Once very hot, add the sliced meat and cook, stirring occasionally, until browned and slightly crispy.
To assemble the gyros, warm the pitas in the oven for a few minutes or in a dry skillet, to make them pliable. Spread each pita with the tzatziki yogurt. Sprinkle with lettuce, tomato slices, onion slices and some of the sautéed meat. Fold over and serve immediately.