Mix rice cereal with 1/3 cup cold water; let stand at least 5 minutes.
Meanwhile, in a 5- to 6-quart nonstick pan, stir half the onion and 1 teaspoon garlic over medium-high heat until limp, about 4 minutes. Stir in tomato paste, broth, and rice. Bring to a boil over high heat, then reduce heat to low, cover, and simmer until rice is tender to bite, 12 to 15 minutes.
As rice cooks, mix beef with 1/2 teaspoon salt and 1/4 teaspoon pepper.
Mix half the beef mixture with one-fourth of the rice cereal mixture, remaining onion and garlic, 1 tablespoon mint, and parsley.
In a food processor, whirl remaining beef mixture and remaining rice cereal mixture until it is a sticky paste.
Shape each of the meat mixtures into 4 oval pieces. In your palm, flatten 1 piece of the beef paste until it's 1/4 inch thick. Lay a coarse-beef oval over the thin piece and fold paste around it, shaping into a 3-inch-long football.
Repeat to form each meatball, and as shaped, drop into simmering broth.
Simmer over low heat, covered, turning meatballs once, until no longer pink in center (cut to test), about 10 minutes.
Add lemon juice, remaining mint, and salt and pepper to taste. Ladle meatball soup into bowls.