Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper. Place the baguette halves cut-side up on the prepared baking sheet.

In a large skillet, heat the butter and olive oil over medium-high heat until the butter is melted and bubbling. Add the diced onions and sauté for about 3 to 4 minutes until they become translucent and fragrant.

Add the finely chopped mushrooms to the skillet along with the garlic powder, salt, and black pepper. Cook for 8 to 10 minutes, stirring occasionally, until all the moisture released by the mushrooms has completely evaporated and they begin to brown.

Spread the cooked mushroom and onion mixture evenly over the cut sides of the baguette halves. Top each half generously with the grated Gouda or Edam cheese, pressing down slightly so the cheese sticks to the mushrooms.

Bake in the preheated oven for 10 to 12 minutes, or until the cheese is completely melted, bubbly, and slightly golden brown. The edges of the baguette should feel crisp to the touch.

Remove the zapiekanki from the oven and let them cool for 2 minutes before slicing into smaller portions. Drizzle generously with ketchup in a zig-zag pattern, sprinkle with fresh chives, and serve immediately alongside a crisp side salad.