Place the sliced onion in the bottom of the slow cooker. Season the roast on both sides with 1 teaspoon kosher salt and 1 teaspoon black pepper, then place it on top of the onion.

Add the beef broth, smashed garlic, and Italian seasoning. Cover and cook on low for 7–8 hours.

Transfer the beef to a large bowl and shred it with two forks. Add some of the reserved slow-cooker liquid to the shredded beef.

To make the cheddar sauce, melt 2 tablespoons butter in a medium saucepan over medium heat. Whisk in the flour to form a paste and cook for 2 minutes.

Slowly add the milk and cook until it reaches a low bubble, not a boil. Add ¼ teaspoon salt and the cheddar cheese. Turn off the heat and stir until the cheese melts.

Toast the onion rolls in a skillet with butter if desired. Fill the rolls with shredded beef and top with cheddar sauce.