Slow Cooker Balsamic Beef and Onions

1 tablespoon olive oil

1 large yellow onion, quartered and sliced

1 ½ pounds boneless beef rump roast

4 cloves garlic, thinly sliced

¼ cup balsamic vinegar, or more to taste

3 tablespoons tomato paste

½ cup water

salt and pepper to taste

Drizzle olive oil into the bottom of a slow cooker, and arrange half the onion slices over the oil. Lay the beef roast on top of the onion, and sprinkle the rest of the onion slices and the garlic around the roast. Sprinkle the balsamic vinegar, tomato paste, and water around the roast. Cover the slow cooker, set to High, and cook until the meat is tender, about 6 hours. Shred the meat into bite-size pieces with two forks, and season with salt and pepper. Serve with the juices from the slow cooker.