Wash beef slices with cold running water. Pat dry with paper towel. Season beef slices with salt and ground black pepper.
In a small bowl, mix marinade – soy sauce, worcestershire sauce and fresh lemon juice.
Transfer beef slices into an airtight container. Pour marinade over seasoned beef slices. Top with red onion rings. Seal container. Refrigerate and allow beef slices to marinate overnight.
In a large skillet, sauté white onions and red onions (from the marinated beef) until translucent. Set aside.
In the same skillet over low heat, pan fry beef slices in batches. Slightly brown beef for about 2 minutes each side. Reserve marinade.
In a bowl, mix 3 tbsp. soy sauce, 1 tbsp. worcestershire sauce, 1 tbsp. lemon juice and brown sweetener (if using). Stir in reserved marinade.
In a pot of the slow cooker, lay some sautéed onion rings, top with beef slices. Repeat layers. Reserve a few pieces of onions for garnishing. Pour soy sauce and lemon mixture. Add beef broth. Slow cook for 4 hours in High heat or until beef is fork tender.