Preheat oven to 180 degrees C. Butter a baking tray.
Cut a hole in the top of each bread round, leaving the sides and bottoms untouched, to make the opening of the bread bowl. Pull apart the cut-out section.
Place 200g butter and half the crushed garlic in a microwave-safe bowl. Heat until butter is melted, 2 to 4 minutes. Brush the insides of the bread bowls and the cut-out pieces with butter; place on the prepared baking tray.
Bake in the preheated oven for 3 to 5 minutes or until crisp. Leave oven on.
Melt the remaining butter in a large saucepan over medium heat. Add onion. Cook and stir until onion has softened and turned translucent, about 5 minutes. Reduce heat to low and continue cooking and stirring until onion is very tender and dark brown, about 20 minutes more.
Stir the remaining garlic, beef broth, chicken broth, wine and Worcestershire sauce into the onions. Bring to a boil. Reduce heat to low; cook until flavors intensify, 5 to 10 minutes. Spoon soup into each bread bowl. Cover soup with Gruyere cheese.
Bake in the preheated oven until cheese is bubbling and melted along the sides of the bread bowl, about 5 minutes. Serve immediately.