Cut the onion in half. Mince one half and slice the other. Remove the eggplant's stem, cut into quarters lengthwise, then halve crosswise. Separate watercress stems and leaves, mince stems and cut leaves into 3cm pieces.

In a plastic bag, combine milk and breadcrumbs, knead lightly. Add ground meat, minced onion, minced watercress stems, salt, and pepper. Knead until sticky. Cut the bag's tip and squeeze out 1/9 portions onto a tray, coat with flour, and shape into balls.

Heat salad oil in a pan over medium heat. Add meatballs and cook for 3 minutes, turning until browned. Add sliced onion and eggplant, stir until coated with oil.

Add all sauce ingredients, mix well, and bring to a boil. Cover and simmer over medium-low heat for 12–15 minutes.