Preheat the oven to 450ºF with the oven rack in the upper third of oven. Line a rimmed baking sheet with heavy duty aluminum foil.
In a medium bowl, mix together the beef, sausage, parmesan, parsley, egg, garlic, panko, salt, and pepper until well combined. Form the mixture into about 40 (½-inch wide) meatballs. Lightly spray the foil-lined baking sheet with nonstick spray and arrange the meatballs in a single layer.
Bake the meatballs until they are firm to the touch, 6 to 8 minutes. Increase the heat to broil. Broil the meatballs until browned, 2 to 4 minutes. Set aside.
In a large stockpot or Dutch oven, heat the oil over medium-high heat. Add the carrots, onion, and celery, and cook until the vegetables start to soften, about 4 minutes. Add the garlic and cook, stirring, for 1 minute.
Gradually stir in the salt, pepper, chicken broth, and beef broth. Bring to a boil. Reduce the heat to a gentle simmer, and add the couscous. Cover and cook 8 minutes or until the couscous is just tender.
Add the spinach and cook until just wilted, 1 minute. Stir in the meatballs and cook until the meatballs are warmed throughout, 2 minutes. Remove the pot from the heat and stir in the lemon juice.
Serve the soup in bowls, and top with additional parmesan cheese, chopped parsley, and black pepper.
