Brown the onion (and garlic) in a soup pot using very little oil. Brown hamburger meat in same pot. Drain off excess fat. Add meat mixture, potatoes, carrots, celery, and tomatoes to a large slow cooker. Cover. cook on high of 1 hour.
Add lentils, seasonings, and stock. Stir. Cover and cook on high 3-4 hours until veggies and lentils are tender. If desired add a 10 ounce package of frozen corn right at the end and stir in.
To freeze: Cool. Divide between 2 or 4 freezer bags (this depends on the size of your family). Remove as much air as possible, seal, label, freeze.
To serve: Thaw Reheat until warmed through.