Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper for easy cleanup.

Melt the butter in a large skillet over medium heat, then add the diced onion and cook until translucent, about 3-4 minutes.

Add the grated or chopped mushrooms to the skillet, seasoning with salt and pepper, and cook until the liquid evaporates and mushrooms are tender, roughly 5-7 minutes.

While the mushrooms cook, slice the baguette in half horizontally to create two long open-faced bases.

Spread the sautéed mushroom and onion mixture evenly over the cut sides of the baguette halves.

Generously sprinkle the grated cheese over the mushroom layer, covering the surface completely.

Place the prepared baguettes on the baking sheet and bake for 8-10 minutes until the cheese is bubbly and the bread edges are golden brown.

Remove from the oven, drizzle generously with ketchup in a zigzag pattern, garnish with fresh chives if desired, and serve immediately while hot.