In a small skillet add a teaspoon or two of olive oil. When hot, add the onions and cook for 3-4 minutes, until softened and translucent. Add the spices: pepper, coriander, cumin, ginger and minced garlic. Cook for an additional minute or so, until the entire mixture is fragrant. Turn off heat and let cool slightly.
To a medium bowl, add the ground lamb, onion and spice mixture, chopped pistachios, fresh parsley, fresh mint and sea salt. Mix with your hands (I like to wear plastic gloves) or a fork until everything is thoroughly distributed. Shape the lamb into 3/4-inch thick patties (about 1/4 pound per patty). Allow the patties to rest and chill in the refrigerator for 15 minutes. (This will make sure the patties will stay together when frying or grilling.)
Meanwhile, mix together the ingredients for the tzatziki sauce. There's no way to mess this up: mix, taste and adjust according to how you like it. Get your toppings ready and warm the pitas in the oven, if you like.
Heat a few tablespoons of olive oil in a medium skillet; when the oil is hot, add the lamb patties, cooking them 1 to 2 at a time. (Over crowding the pan will result in steamed patties.) Cook each of the patties for about 3-4 minutes per side for medium well. If you like your lamb well, then I'd recommend about 5 minutes each side. Repeat the process until all the patties are cooked.
To assemble, place the lamb patties in the pitas, garnish with tomatoes and lettuce. Pour on tzatziki sauce liberally. It's good.