Optional Browning (this step improves flavor and helps the beef and helps it to stay tender, but can be skipped in a pinch): Heat 2 Tbsp cooking oil (not in ingredients list) in a large skillet or saute pan over medium meat. Pat the beef dry and season it lightly with some salt and pepper. Add the beef to the skillet and cook on all sides until brown. It does not need to be cooked through since it will finish cooking in the slow cooker.
Combine onions, celery, carrots, garlic, tomato paste, diced tomatoes (including liquid), broth, barley, beef stew meat, Italian seasoning, and bay leaves in the bowl of a slow cooker. (See below for freezer instructions.)
Cook on low for 7 to 8 hours. (Note: I don’t recommend cooking this on high because many slow cookers so hot on the high setting that they can boil, leading to tougher, drier pieces of meat.)
Discard bay leaves.
Stir in parsley and lemon juice.
Taste soup and season with some salt and pepper if needed.
Serve with warm baguette slices.