In a large mixing bowl, combine ground beef, grated onion, minced garlic, one egg, breadcrumbs, mint, parsley, oregano, salt, and pepper.
Knead the mixture thoroughly for 2-3 minutes to ensure flavors are well incorporated and the texture is light.
Shape the meat mixture into 1.5-inch (4 cm) balls.
In a wide skillet, heat olive oil over medium-high heat and sear the meatballs until browned on all sides.
Pour the chicken broth into the skillet until meatballs are half-submerged, reduce heat to low, cover, and simmer for 20 minutes.
In a medium bowl, whisk two eggs and lemon juice until frothy; whisk in the cornstarch if a thicker sauce is desired.
Temper the egg mixture by slowly whisking in two ladles of the hot cooking broth from the skillet to prevent curdling.
Pour the tempered egg-lemon mixture back into the skillet with the meatballs.
Gently shake the pan or stir over low heat for 2-3 minutes until the sauce thickens into a velvety consistency, ensuring it does not reach a boil.
Serve immediately, garnished with additional fresh herbs.