In a large skillet over medium-high heat, add olive oil. Season the pot roast with salt and pepper on all sides. When the oil is hot, sear the roast for 3-4 minutes on each side until it develops a deep brown crust. Remove it from the skillet and set it aside.
In the same skillet, add the sliced onions and cook over medium heat until they are soft and caramelized, about 15-20 minutes. Stir in the minced garlic and cook for an additional 2 minutes.
In the slow cooker, place the caramelized onions and garlic evenly at the bottom.
Place the seared pot roast on top of the onions in the slow cooker.
In a mixing bowl, combine beef broth, balsamic vinegar, thyme, bay leaves, salt, and pepper. Stir well and pour the mixture over the roast in the slow cooker.
Cover and cook on low for 8-10 hours or on high for 4-6 hours, or until the roast is tender and easily shreds with a fork.
In the last 30 minutes of cooking, place the slices of cheese on top of the roast, allowing it to melt.
Once cooked, remove the roast from the slow cooker and shred it with two forks. Return it to the slow cooker, mixing it with the onions and broth.
Serve the pot roast hot in bowls, garnished with fresh parsley.