Massage the kosher salt into the chuck roast. Set aside.
Add the brown sugar, mustard, soy sauce, water and OLIPOP cola into a large bowl or pan. Whisk the ingredients together until the brown sugar is dissolved.
Add the chuck roast to the marinade. Flip the chuck roast a few times in the marinade to be sure it is coated. Cover and refrigerate overnight.*
After marinating, remove the meat from the refrigerator and let it sit at room temperature for about 15 minutes.
Preheat the oven to 325ºF and heat the avocado oil in a large Dutch oven over medium/high heat.
Remove the meat from the marinade and let any excess marinade drip from the meat. Sear the meat for 3-5 minutes on each side, until a crust forms on each side.
Remove from the heat and pour the marinade into the Dutch oven. Add the garlic and onion to the Dutch oven and cover.
Transfer the meat to the oven and bake for 3.5 - 4.5 hours, flipping the meat once to ensure both sides are submerged in the marinade.
The roast is done when it easily falls apart when you try to shred it with a fork or when it has an internal temperature of 205ºF - 210ºF.
Remove the roast from the oven and shred the meat. Toss it with the marinade. Serve on tortillas, buns, or over mashed potatoes.