In a large bowl, combine the chicken thighs, soy sauce, and minced garlic. Marinate for at least 30 minutes, or up to 2 hours in the refrigerator for more flavor.
Heat the vegetable oil in a large skillet or Dutch oven over medium heat. Remove the chicken from the marinade (reserve the marinade) and brown the chicken on all sides, about 5-7 minutes.
Add the sliced onion to the skillet and cook until softened, about 3 minutes.
Pour in the reserved marinade, vinegar, water, bay leaves, black peppercorns, and brown sugar. Stir to combine.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 30-40 minutes, or until the chicken is tender and cooked through.
Taste and adjust seasoning with salt if needed. If you prefer a thicker sauce, remove the lid and simmer for an additional 10-15 minutes.
Serve the chicken adobo hot, garnished with chopped green onions over steamed rice.