Heat a large skillet over medium-high heat. Once hot, add oil, and once oil is hot, arrange lamb in a large even patty about ¼-inch thick. Season with salt and pepper. Cook until underside and browned and very crisp, about 5 minutes.
Carefully pour off most of excess fat and discard. Flip meat in spatula-sized parts, sprinkle top with garlic, a good pinch of red pepper flakes (up to 1 teaspoon for real heat; I used ¼ teaspoon), cumin and coriander, plus more salt and pepper, and cook until underside is browned, which should only take 2 to 3 more minutes.
Use spatula to break lamb into smaller chunks and add lentils. Cook, stirring occasionally, until lamb is cooked through, about 2 minutes more. Transfer lamb-lentil mixture to a bowl.
Arrange lettuce leaves on a platter and divide lamb-lentil mixture between them. Top with yogurt, tahini, or yogurt-tahini sauce, tomato-cucumber relish and herbs. Serve with hot sauce, if desired.
