Preheat oven to 375 degrees.
Place cut potatoes into a large pot and fill with water so that it is just covering potatoes by 2 inches. Bring to a boil.
Once boiling, add 1 tbsp of the salt and continue to boil until potatoes are fork-tender, about 10 minutes.
Meanwhile, heat a large skillet over medium heat with 2 tbsp of the butter. Add onion and garlic then cook, stirring, until the onions are tender, 4 to 6 minutes.
When potatoes are tender, strain and let steam escape a the colander for about 5 minutes, allowing excess water to drain [this prevents the potatoes from not tasting watery].
Place potatoes in a large bowl and add the sour cream, white vinegar, chicken broth, pepper, 1/4 cup of the chives, and 2 tbsp of butter.
Using a potato masher, mash until potatoes are your desired consistency. [For this recipe, I prefer mine a little lumpy.]
Once mashed, stir in the onion mixture with black pepper and 2 tsp of kosher salt.
Add the potatoes into an oven-safe dish and sprinkle with cheese.
Place into the oven, uncovered until cheese is lightly browned, 10 to 15 minutes.
Once browned, top with remaining chives, serve and enjoy!