Place the chicken, balsamic vinegar, soy sauce, chicken stock, rosemary, and black pepper into a Dutch oven or non-reactive pot and marinade in the fridge for at least 6 hours but ideally overnight. Remove the pot from the fridge 1 hour prior to cooking.
Preheat oven to 450°F. Toss the potatoes with 3 tablespoons olive oil, and salt and pepper to taste. Place the potatoes in a large baking dish and roast in the oven until almost fork tender, about 30 minutes.
While the potatoes are roasting, place the pot of chicken and marinade onto a burner and bring to a boil. Once boiling, turn the heat to low and simmer for 45 minutes.
Carefully remove the chicken thighs onto a plate, saving the residual braising liquid. The rosemary can be discarded.
Heat a large pan to medium heat with the remaining olive oil and add the onion along with a pinch of salt. Cook until soft then add the garlic and continue to cook for another 2-3 minutes or until golden.
Push the onion and garlic to the side of the pan and add the chicken skin side down and sear for a few minutes or until the skin begins to brown. Turn the chicken skin side up, and add the remaining braising liquid and turn the heat to high. Boil until the sauce reduces by about half and coats the chicken then turn off the heat.
Place the chicken on top of the potatoes. The sauce can be degreased by spooning some of the fat that rises to the top or by using a couple of paper towels to absorb the top layer of fat. Pour the sauce all over the potatoes. Return to the oven and cook for another 10-15 minutes or until the potatoes are fork tender. Taste test and season with salt and pepper if required. Enjoy!