Remove the chuck roast (or roasts) from their packaging and use a fork to stab a few holes on all sides.
Whisk together the olive oil and red wine vinegar then rub it all over the meat.
Sprinkle evenly with salt, pepper, parsley, basil, and garlic powder on all sides.
Place half of the onion on the bottom of the slow cooker followed by the roast and all of the drippings from the cutting board. Sprinkle with garlic cloves and top with the remaining onion.
Cover and cook on low for 8-10 hours or on high for 4-5 hours, until the beef shreds easily with a fork. If you check it after 8 hours and it's not easy to pull apart, continue cooking for another 1-2 hours.
Remove the roast from the crock pot and transfer to a cutting board. Pull the beef apart using two meat forks. Beef can be served immediately with juices over noodles or mashed potatoes. It also makes a great sandwich, taco, or burrito!