Finely chop parsley to equal 1/2 cup and mint to equal 2 tablespoons. Place in a large bowl with meat, egg whites, rice, crumbs, salt and 2 tablespoons of water. Blend well. Shape mixture into a 1.5 inch thick square, cut into 16 pieces and roll each into a ball.
In a small bowl, blend cornstarch with 3/4 cup water. Place in a large skillet over medium heat with chicken broth and bring to a simmer. Add meatballs and cook, covered, for 15 minutes, turning meatballs occasionally. Remove meatballs to a serving dish and keep warm. Reduce heat to low.
Squeeze lemon to yield 2-3 tablespoons of juice. In a small bowl, whisk egg yolks with lemon juice. Stirring constantly, add 3 tablespoons of the hot broth to the yolk mixture. Gradually add back to the skillet, stirring constantly. (NOTE - do NOT skip this step; it tempers your egg yolks so that you don't end up with scrambled eggs in your sauce!).
Simmer for 2 minutes or until sauce thickens; do not boil. Pour over meatballs and serve immediately.