Add the beef to the slow cooker.
Combine 1/4 cup olive oil, garlic, oregano, salt, pepper, and lemon juice in a small bowl. Pour over the meat and toss to coat, in the slow cooker. Toss in the sliced onions.
Cover and cook on LOW for 6-10 hours (depending on your slow cooker/crock pot) or until beef is extremely tender (If it’s not tender, it’s not done yet).
Remove lid and salt and pepper to taste. If desired, break up the meat into smaller bite size pieces.