Caramelize the onions

• Melt the butter with the olive oil in a Dutch oven over medium-low heat.
• Add the sliced onions with a pinch of salt and cook slowly, stirring often, until deep golden brown.
• Add the garlic and thyme and cook until fragrant.

Deglaze and build the onion base

• Pour in the white wine and scrape up the browned bits.
• Add the beef stock and simmer until slightly reduced.
• Season with salt and pepper.

Cook the pasta

• Boil the macaroni in salted water until just shy of al dente.
• Drain and set aside.

Make the cheese sauce

• Melt the remaining butter in a saucepan and whisk in the flour.
• Gradually whisk in the milk and cook until thickened.
• Off the heat, stir in most of the Gruyère, the mozzarella and the Parmesan until melted.

Combine and bake

• Stir the pasta and onion mixture into the cheese sauce and transfer to a baking dish.
• Top with the torn baguette pieces and the remaining Gruyère.
• Bake at 200 °C until bubbling and golden on top.