Heat the olive oil in a large skillet. Generously season the beef with salt and pepper. Add it to the skillet and sear it so it is brown on all sides. Add it to the base of your crock-pot. If the meat is too big for the bottom of the crock, you may choose to cut it into 3-5 pieces before you sear it.
Into the oil, add the garlic and onion and cook for 2 to 3 minutes until soft. Add that into the crock pot on top of the meat.
Next, add the vinegar and tomatoes to deglaze the pan. Make sure to scrape the bottom of the pan to get all those flavor bits off the bottom and into the braising liquid. Then add the remaining spices and beef or chicken stock and stir well to combine. Pour this over the top of the beef inside the crock pot.
Cover and cook on LOW for 6 hours or until the beef is tender enough to shred.
Remove the beef and shred it with 2 forks. Place the meat onto a serving dish.
Transfer the broth to a sauce pan (or your crock-pot insert the stovetop - if it's able) and bring to a boil.
Mix the cornstarch and 1/4 cup of cold water stir until smooth. Pour into the sauce and whisk while boiling until the sauce is thickened. Adjust salt and pepper to taste.
I toss the beef in the sauce to get it fully covered. You may like to serve the sauce on the side.