Using a sharp knife, make shallow incisions on all sides of the chuck roast to create holes for seasoning.
In a small bowl, whisk together olive oil and red wine vinegar until well combined. Rub this mixture evenly over the entire surface of the roast.
Sprinkle seasoning salt, dried basil, dried parsley, and garlic powder evenly over both sides of the meat.
Place half of the sliced onion in the bottom of the slow cooker. Add the seasoned chuck roast. Layer the remaining onion and sliced garlic on top of the roast.
Cover and cook on low for 8 to 10 hours, or until the beef is tender and easily shreds with a fork.
Once the cooking time is complete, carefully remove the roast from the slow cooker and transfer it to a cutting board. Using two forks, pull the beef apart into shreds or large pieces. Serve alongside your favorite sides or incorporate into other dishes as desired.