Stir the garlic, horseradish, hot pepper sauce, thyme, salt, pepper, Cajun seasoning, olive oil, and malt vinegar together in a bowl until thoroughly blended.
Pierce the beef roast all over with a meat fork. Place the roast in a large, resealable plastic bag. Spoon in the marinade and turn the roast so it's well coated. Refrigerate overnight, turning occasionally if desired.
When ready to cook, place the roast in a slow cooker along with any remaining marinade. Do not add water. Roast on Low for 8 to 10 hours, or until desired doneness. For medium-rare, a meat thermometer should read 135 degrees F (57 degrees C). Remove from the slow cooker to a serving plate, and allow to rest 15 minutes before slicing across the grain.
