Pat the chicken drumsticks dry with a paper towel and place them into a large, sealable plastic bag. Set aside.

In a medium glass*, whisk the balsamic vinegar, olive oil, soy sauce, chicken stock, red pepper flakes, and minced garlic together. Season with salt and pepper, if desired.

Pour one-half of the balsamic marinade over the chicken drumsticks and seal the bag tightly. Turn the bag over until the chicken is coated. Place the bag in the refrigerator for a minimum of 30 minutes and up to overnight. Reserve the remaining marinade to use later.

When ready to cook, preheat the oven to 400°F/200°C/Gas 6 and line a large, rimmed baking sheet with parchment paper.

Remove the drumsticks from the plastic bag and arrange them on the lined baking sheet in a single layer without overcrowding. Discard the remaining marinade inside the bag.

Place the chicken into the preheated oven for 30 minutes.

Remove the chicken from the oven and turn each drumstick with tongs. After turning, brush the tops of each drumstick with the remaining marinade and return to the oven. Bake for another 20-25 minutes or until the chicken is fully cooked, the juices run clear, and the outside has caramelized.

Remove from the oven and let the chicken rest for 5 minutes.

Transfer the drumsticks to a serving dish and sprinkle them with sliced green onions before serving.