Pat chicken drumsticks dry with a paper towel and place in a large, sealable plastic bag. Set aside.
In a medium glass or other non-reactive bowl, whisk the balsamic vinegar, olive oil, coconut aminos, chicken stock, red pepper flakes, and minced garlic together. Season with salt and pepper, if desired.
Pour one half of the balsamic marinade over chicken and seal bag tightly. Turn bag over until chicken is coated. Place bag in refrigerator for a minimum of 30 minutes to overnight. Reserve remaining marinade to use later.
When ready to prepare, preheat oven to 400° and line a large rimmed baking sheet with parchment paper. Remove drumsticks from plastic bag and arrange on lined baking sheet in a single layer without overcrowding. Discard remaining marinade inside bag.
Place baking sheet in preheated oven for 30 minutes, then remove chicken from oven and turn each drumstick. After turning, brush the tops of each drumstick with remaining marinade and return to oven. Bake for another 20-25 minutes or until the chicken is well done and the outside has caramelized.
Remove from oven and let rest for 5 minutes. Transfer drumsticks to a serving dish and sprinkle with sliced green onions before serving.