1. Stir together 4 cups marinara sauce and 1 tsp. harissa paste in a 6-quart slow cooker. In a large bowl, combine 1 lb. ground lamb, 1 lb. lean ground beef, 1 egg, 1 1/2 tsp. each ground garlic, cumin, black pepper, coriander, and paprika, and 1/4 tsp. kosher salt. Form 2-inch diameter meatballs (about 20 meatballs) and nestle into the sauce (be careful not to mash them). Cook until meatballs are no longer pink in the center and reach 165 degrees when tested with an an instant read thermometer: 2 1/2 hours on high, 5 hours on low.
2. Spoon half of the meatballs and sauce into bowls and garnish with chopped fresh mint and plain Greek yogurt (optional).Eat It Again!
3. Transfer the remaining meatballs and sauce to a storage container. Cool, and then refrigerate for up to three days or freeze for up to three months. Reheat from the fridge on the stove-top or in the microwave. If frozen, defrost overnight in the fridge and then heat on the stove-top or in the microwave.Make It a Meal
4. Serve these saucy, Moroccan-ish meatballs with pita bread or on top of couscous or even gnocchi.