In a large mixing bowl, combine the ground lamb and ground beef. Add the minced garlic, chopped cilantro, chopped parsley, ground cumin, ground coriander, paprika, ground cayenne pepper, ground black pepper, ground fennel seeds, salt, ground cinnamon, and harissa paste (if using). Mix thoroughly until all ingredients are well combined.
If using casings, soak them in warm water for about 30 minutes. Rinse thoroughly and drain.
Stuff the meat mixture into the casings using a sausage stuffer or a piping bag. Twist the sausages into 4-6 inch links.
If not using casings, you can shape the meat mixture into small, thick logs or patties.
Preheat your grill to medium-high heat or heat a large skillet over medium-high heat.
Grill or fry the merguez sausages for about 5-7 minutes on each side, or until they are cooked through and browned all over. If grilling, you can also cook them over indirect heat to prevent burning.
Enjoy your meal!