Preheat your oven to 200°C.

Slice the baguette in half lengthwise.

In a pan, melt the butter over medium heat. Add finely chopped onions and sauté until translucent.

Add sliced mushrooms and minced garlic to the pan. Cook until the mushrooms are soft and slightly browned.

Season the mushroom mixture with salt and black pepper to taste.

Spread the mushroom mixture evenly over the sliced baguette halves.

Top with a mixture of grated mozzarella and cheddar cheese.

Place the baguette halves on a baking sheet and bake in the preheated oven for about 10-15 minutes, or until the cheese is melted and bubbly.

Once baked, remove from the oven and let it cool for a couple of minutes.

Garnish with finely chopped chives and drizzle with ketchup just before serving.