Pulse 1 cup cilantro with the scallions in a mini food processor to make a coarse puree. Transfer half of the mixture to a large bowl and add the ground beef, egg, 1 tablespoon harissa, 1/2 teaspoon cumin and 1/2 teaspoon salt. Mix with your hands until just combined. Form into twenty 1-inch meatballs.
Heat the olive oil in a large pot over medium heat. Add the carrots and cook, stirring, until they start browning, 4 to 5 minutes. Stir in the remaining cilantro puree, 1 tablespoon harissa and 1 teaspoon cumin; cook until sizzling, about 1 minute. Stir in the chicken broth and 2 cups water; bring to a simmer and cook 2 minutes. Gently drop in the meatballs and simmer until almost cooked through, 7 to 8 minutes.
Add the spinach and couscous to the soup and cook until the couscous is tender, about 5 minutes. Stir in the remaining 1/2 cup cilantro and season with salt. Sprinkle with more cumin before serving.