Add the cumin, coriander, oregano, paprika, red pepper flakes, cinnamon and 1 1/2 teaspoons salt to a small bowl and stir to combine. Add the ground lamb to a large bowl and break up slightly with a fork. Add the spice mixture to the lamb and using clean hands, mix the meat so the spice mixture is evenly dispersed. Form the meat into 4 equal size patties to fit the size of the buns.
Brush the grates of an outdoor grill with oil and preheat to medium-high. On one side of the grill add the burgers and cook until the internal temperature is 145 degrees F, about 4 minutes per side. Remove the burgers to a plate. Once you remove the burgers, place the buns, cut side- down on the grill to lightly toast, about 2 minutes.
To assemble burgers: Lay buns on a clean work surface, cut side-up. Spread the Tzatziki Sauce evenly on the top and bottom halves of the buns. Place a patty on each of the bottom buns and top with Charred Tomatoes and Feta. Place the top buns on and serve.
Yield:About 1 cup
Grate the cucumber on the large holes of a box grater. Using clean hands, squeeze as much liquid as possible out of the cucumber. Add it to a small bowl and stir in the sour cream, dill, lemon juice and 1/4 teaspoon salt. Refrigerate in an airtight container for up to 3 days.
Charred Tomatoes and Feta:
Yield:About 3/4 cup
Add the tomatoes to a large bowl and place the onions on top. Add the olive oil and 1/4 teaspoon salt and, using clean hands, rub the onions so they are coated in olive oil, but try to keep them intact in their rings. Lay the onions on a plate or rimmed baking sheet. The residual salt and oil will have dripped down to the tomatoes, so give them a slight toss to coat them. Skewer the tomatoes on a metal or wood skewer. This will help them not fall through the grates of the grill. (If using a wood skewer then be sure to soak in water for 30 minutes before using.)
Brush the grates of an outdoor grill with oil and preheat to medium-high. Add the onions and tomatoes and grill until the onions are charred in places and the tomatoes are slightly bursting, about 2 minutes per side. Remove the onions and tomatoes and add to a medium bowl. Using kitchen shears, roughly cut up the onions and tomatoes. Add the crumbled feta and toss to combine. Refrigerate in an airtight container for up to 3 days.