1 lb. trimmed lamb shoulder, cut into 1" cubes, or 1 lb. ground lamb
3 cloves garlic, finely chopped 1 tbsp. harissa
1 tbsp. minced flat-leaf parsley
1 tsp. paprika
1⁄2 tsp. ground coriander
1⁄2 tsp. ground cumin
1⁄4 tsp. ground fennel seed
Kosher salt and freshly ground black pepper, to taste
4 tbsp. extra-virgin olive oil
1⁄2 cup yogurt
8 leaves basil, roughly chopped
Chopped tomatoes, red onions, and cucumbers, for serving
Flat bread, for serving
Put lamb into the bowl of a food processor fitted with the chopping blade and transfer to freezer to let chill for 30 minutes. Add 2 cloves garlic, harissa, parsley, paprika, coriander, cumin, fennel, salt, and pepper to the bowl; process until lamb is coarsely chopped and mixed with spices, about 15 seconds. (If using ground lamb, just mix lamb with other ingredients in a large bowl.) Divide lamb mixture into 8 portions and form the portions into 3"-wide patties. Heat 2 tbsp. oil in a 12" cast-iron skillet over medium-high heat. Add lamb patties and cook, turning once until browned and still slightly pink, about 6-8 minutes. Transfer merguez to paper towels; set aside.
Meanwhile, stir together remaining garlic and oil, yogurt, and basil in a small bowl to make a sauce; season with salt and pepper. Serve merguez on a platter with sauce, chopped vegetables, and flat bread.