Preheat your oven to 400°F (200°C).

Melt the butter in a large skillet over medium heat.

Add the diced onion and sauté for about 3-4 minutes until it becomes translucent and soft.

Add the chopped mushrooms to the skillet. Cook for 10-12 minutes, stirring occasionally, until the mushrooms have released their liquid and it has completely evaporated. The mixture should be relatively dry.

Season the mushroom mixture with salt and black pepper, then remove from heat.

Cut the baguette in half lengthwise to create two long open-faced halves. If the baguette is too long for your oven, cut it crosswise first.

Spread the mushroom and onion mixture evenly over the cut side of the bread.

Generously cover the mushrooms with the grated cheese.

Place the zapiekanki on a baking sheet and bake for 10-12 minutes, or until the cheese is bubbly and golden and the bread is crispy.

Remove from the oven and immediately drizzle with ketchup in a zig-zag pattern before sprinkling with fresh chives to serve.