1. 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.
2. 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.