Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper. Hollow out the soft centers of the baguette halves slightly to make a shallow boat for the filling.

Heat the butter and olive oil in a large skillet over medium-high heat. Add the diced onion and sauté for 3-4 minutes until softened and slightly translucent.

Add the finely chopped mushrooms and minced garlic to the skillet. Cook for 8-10 minutes, stirring occasionally, until all the moisture released from the mushrooms has completely evaporated and they begin to turn golden brown.

Season the mushroom mixture with salt and black pepper, then remove the skillet from the heat. Divide the mixture evenly among the four baguette halves, pressing it gently into the hollowed bread.

Top each mushroom-loaded baguette generously and evenly with the shredded Gouda cheese.

Place the baguettes on the prepared baking sheet and bake for 10-12 minutes, or until the cheese is completely melted, bubbly, and the edges of the bread are golden and crispy.

Remove from the oven and let cool for 1-2 minutes. Drizzle the tops with ketchup in a classic zigzag pattern, sprinkle with fresh chives, and serve immediately while hot and crispy.