Place the beef stew meat in a large bowl and toss it with the flour, salt, and pepper until evenly coated.
Add the coated beef, carrots, potatoes, onion, celery, garlic, thyme, and bay leaf to a freezer bag. Seal the bag and freeze flat for later use.
When ready to cook, thaw the bag overnight in the fridge. Pour the contents into the slow cooker and add the beef broth along with the diced tomatoes.
Stir gently to combine, then cover and cook on low for 8 hours until the beef is fork tender.
Remove the bay leaf before serving.