Caramelize the onions

• Melt the butter with the avocado oil in a large pot.
• Add the onions, green apple and garlic and cook low and slow, stirring often.
• When the onions stick, add a splash of water and scrape up the browned bits; repeat until deep brown and caramelized.

Add the flour

• Stir the flour into the caramelized onions and cook for about a minute.

Simmer the soup

• Pour in the beef broth and add the grated bouillon cube, salt, pepper, thyme and bay leaves.
• Simmer and reduce for a while, then remove the thyme and bay leaves.

Make the grilled cheese

• Layer the gouda, gruyère and mozzarella between two slices of sourdough.
• Butter the outside and cook on medium-low until golden and crispy on both sides and the cheese is melted.

Broil the soup topping

• Ladle the soup into oven-safe bowls and top with toasted sourdough and shredded mozzarella.
• Broil until the cheese is bubbling and golden.