Melt the ghee in a large frying pan over medium heat. Add the onions and cook for 15-20 minutes, sturring occasionally. Once the onions are tender, strain the ghee directly into your slow cooker using a fine-mesh sieve double lined with cheesecloth.
Add the leeks, wine, beef broth, Worchestershire sauce, salt and thyme to the slow cooker and cook on low for 4.5 hours.
Once the soup is cooked, preheat your broiler on high for 5-10 minutes. Combine the garlic-infused oil and basil in a small bowl and brush it generously onto your bread cubes. Broil the bread until the edges are golden brown and crunchy (about 3 – 5 minutes)
Ladle your soup into 4 oven-safe bowls. Top each bowl with bread and Swiss cheese. Place the bowls in the oven and broil them until the cheese has melted (about 2-4 minutes). Serve immediately.