The first step is to turn the garlic into a paste. You can do this by pulsing it in a food processor, using a garlic press, or by mincing it and then pressing it into a paste using the back of your knife and a little coarse salt.
Add the beef broth and onions to the slow cooker. Place tri-tip on top. Cook on low for 8 hours. Thirty minutes before the cooking is finished, remove the lid and shred the meat. Let cook, uncovered, for the final thirty minutes in the liquid.