Brown the Beef: Heat the olive oil in a large skillet. Add the ground beef and chopped onion, then season with half of the salt, pepper, and minced garlic. Cook until the beef is done. Build the Soup: Transfer the cooked beef and onion mixture to the bottom of the slow cooker. Add the chopped carrots and celery, stirring them together with the beef mixture. Add the Liquid: Gently pour the crushed tomatoes and stock into the slow cooker. Add the drained cannellini beans, remaining salt and pepper, and dried thyme.
Combine the Ingredients: Stir everything together gently until the beef, vegetables, beans, tomatoes, stock, and seasonings are evenly distributed.
Slow Cook the Soup: Cover the slow cooker and cook on low for 5 hours, allowing the vegetables and other ingredients to develop their flavors.
Add the Corn: Remove the lid and stir in the frozen corn. Continue cooking for 1 additional hour. Finish and Serve: Stir the finished soup thoroughly, taste and adjust the seasoning as needed, then serve hot.