Heat the broiler to high. Combine the bread and milk in a medium bowl and stir to combine. Add the lamb, coriander, garlic, 1 teaspoon salt, and ¼ teaspoon pepper. Form into 50 teaspoon-size balls. Place on a rimmed baking sheet and broil 3 minutes. Remove from the oven.
Meanwhile, cook the Israeli couscous according to the package directions; reserve.
Cook the onions, harissa, and the remaining salt and pepper in the olive oil in a large pot over moderate heat, partially covered, until the onions are softened, 8 minutes. Add the tomatoes, broth, and meatballs and simmer for 15 minutes.
Stir in the couscous to warm and serve, garnished with mint.