Place sliced red onion on bottom of slow cooker. Generously season roast with Kosher salt and freshly ground pepper and place on top of onion in slow cooker.
Finely chop chipotle peppers and add to slow cooker, along with adobo sauce, root beer, brown sugar, garlic powder, and paprika. Cook on low for approximatley hours, until meat is very tender and falling apart.
Remove meat to a bowl and shred using two forks. Remove any excess fat.
Create a slurry by stirring cornstarch in water until dissolved. Pour into slow cooker and stir to incorporate. Return shredded beef to slow cooker and cook for an additional 1 hour to allow flavors to meld.
If the sauce is still too thin, remove meat and onions and pour liquid into a small saucepan. Bring to a simmer over medium-low heat and allow to reduce to desired consistency. I did this to concentrate the flavor of the sauce a bit and to allow it to thicken to a glaze like consistency. Combine sauce with shredded beef.
Serve with all the fixings for tacos, or make quesadillas with shredded cheddar and monterey jack cheese and pickled onions.