In a pan over medium heat, add in 2 tbsp of butter, thinly sliced onions and salt. Add in the beef broth and let it cook while stirring until brown and caramelized.
Set aside when cooked.
Pre-heat the oven to 180 degrees.
In a bowl, add in the unsalted butter, minced garlic, salt, pepper, chili flakes and parsley. Mix well till all combined.
Spread the garlic paste over the brioche bread and broil until slightly golden.
Take it out and spread the French onion mixture and mozzarella cheese. Broil again until the cheese is melted.
Serve while warm!