Season both sides of the short ribs with the salt and the pepper.
In a heavy-bottomed skillet over medium-high heat, heat the oil.
Cook the short ribs, in batches, until they are browned on all sides, about 10 minutes per batch.
Transfer the short ribs to a large, lidded plastic container.
In the same skillet, add 1 1/2 cups of the room temperature water, the cider vinegar, and the red wine vinegar
Bring the marinade mixture to a simmer and cook, while stirring, loosening the browned bits off the bottom of the pan.
Add the juniper berries, the cloves, the bay leaves, the salt, and the pepper to the marinade mixture.
Transfer the skillet from the heat and add 1 cup of the cold water. Stir to combine.
Pour the marinade over the short ribs and place the plastic tub in an ice bath until chilled, about 30-40 minutes.
Cover the plastic tub with the lid and allow the short ribs to marinate in the refrigerator for 24 hours.
In a large, heavy pot over medium heat, melt the butter.
Stir the onion, the celery, and the carrot into the melted butter and cook until the veggies are softened, about 10 minutes.
Add the garlic to the veggies and cook until fragrant, about 1 minute.
Place the marinated short ribs over the top of the veggie mixture.
Pour the remaining marinade, the remaining room temperature water, the chicken broth, and the sugar over the short ribs and bring to a simmer over medium-high heat.
Reduce the heat to low and simmer the short ribs, covered, until they reach an internal temperature of 200-205 degrees F, about 3-4 hours.
Transfer the short ribs to a platter.
Remove and discard the juniper berries, the cloves, and the bay leaves.
Sprinkle the cooked veggie mixture over the top of the short ribs, reserving the excess cooking liquid.
Place the pot with the remaining cooking liquid over medium-high heat.
Place the gingersnaps in the food processor and grind until they are fine.
Add the grinded gingersnaps and the balsamic vinegar into the cooking liquid and boil, while skimming the fat, until it reduces and thickens, about 20 minutes.
Season the gravy with the salt.
Strain the gravy through a fine mesh sieve.
Serve the short ribs over the mashed potatoes, drizzled with the gravy.