Preheat oven to 325°F.
Cut the chuck roast into several large chunks. Add salt, pepper, and flour to the meat in a gallon-size resealable bag. Shake to mix. Add roast chunks to the bag. Close and shake until each side of the roast pieces is completely coated with flour seasoning.
In a 6-quart Dutch oven, add olive oil and heat on high heat. When hot, sear the coated roast chunks for 2 minutes on each side, making at least 4 turns, about 8-10 minutes. Remove the roast chunks to a plate.
In the same Dutch oven, over medium heat, add butter, thinly sliced onions, 1 teaspoon of salt, 1 teaspoon of black pepper, and 1 teaspoon of sugar. Cook for 2 minutes, stirring to distribute the butter and seasoning to the onions. Then cover with a lid and stir occasionally for 10-15 minutes, until the onions are tender. Add the garlic and continue to cook, uncovered, for an additional 10-15 minutes, until the onions are browned and caramelized.
Center roast chunks over the onions and in the center of the pot. Drizzle meat with balsamic vinegar and Worcestershire sauce.
Add the bay leaves, thyme, and sage. Add peeled potatoes to the edges of the pot. Pour beef broth and apple cider vinegar over the potatoes (not the meat) until the meat is halfway submerged. Cover and place in the hot oven. Braise for 3 to 4 hours, or until the meat is tender and falls apart easily with a fork. If the fork easily pierces the meat and it falls apart, then it is done. If the meat is not tender, re-cover the pot and continue to cook until the meat tests tender
While the roast is braising, prepare the garlic for roasting by cutting off the top of the garlic and drizzling it with olive oil. Wrap the entire garlic head in foil, with the opening at the cut top. Double wrap to be sure the oil does not leak. After the roast has been in the oven for 2 hours, add the foil-wrapped garlic to the oven, to the side of the roasting pot. You will roast the garlic in the same 325°F oven for 50-60 minutes, or until tender.
Remove the garlic from the oven and set it aside until you are ready to mash the potatoes. When the roast is tender, remove the pot from the oven and turn the oven temperature up to 425°F. Remove the potatoes to a large bowl, mash, and cover with a lid. Set aside. Add additional broth to the pot if necessary to keep the onions covered. Place the pot back in the oven for 20 to 30 minutes, or until the meat is caramelized on top.
In a small saucepan, add roasted garlic, 1/3 cup butter, and ¾ cup of heavy cream. Heat on medium-high heat until warmed enough to smash the garlic into the liquid. Add a third of the butter/garlic liquid to the mashed potato bowl, and with a hand mixer, mix the liquid into the potatoes. Continue adding a third of the liquid each time, mixing the potatoes until they reach a thick, creamy consistency. Add garlic powder, salt, and pepper to taste. Cover and set aside until ready to serve the roast.
Remove the roast from the pot, shred (or cut), and set aside. If needed to thicken the gravy and onions, add 2 Tablespoons of cornstarch to a small bowl and add enough water to make a slurry. Stir with a spoon until smooth. Stir while adding it slowly to the pot of gravy, and continue stirring over medium heat until the mixture has thickened. Mix the shredded roast back into the gravy and onions. Serve over mashed potatoes. Optionally garnish with fresh thyme leaves.