For an even richer flavor, brown the ground beef in batches to avoid overcrowding the pot. Feel free to substitute other vegetables, such as celery, bell peppers, or spinach, based on your preferences and what you have on hand. To make this soup even more convenient, it can be easily prepared in a slow cooker. Simply brown the beef and sauté the vegetables as directed, then transfer everything to the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours. This soup tastes even better the next day, as the flavors have had time to meld together.