In a large mixing bowl, mix together the ground lamb, mint, salt, pepper and egg. Using clean hands, mix until well combined. Measure 1 tablespoon of the meat mixture and roll in the palm of your hands to form a meatball. Place the formed meatballs on a parchment-lined baking sheet. You should have enough to make 16 to 18 meatballs.
In a heavy saucepan, heat the vegetable oil on medium-high heat. Add the onions and tomato paste and cook, stirring, until the onions are softened and the tomato paste is cooked into the onions, about 3 minutes. Add the celery, corn and carrots. Saute until soft, about 2 minutes. Add the chicken broth. Bring to a boil, and then add the lamb meatballs (albondigas) and the bay leaf. Score the serrano chile by finely slicing it down the middle, being careful not to fully cut it in half, and add to the pot. Lower the heat and cover. Simmer for 35 minutes.
Using a large spoon, skim the grease off the top of the soup and discard. Season the soup with salt and pepper.