Poland's street-food classic: half a baguette under mushrooms and melted cheese, baked until the edges crisp and finished with a zig-zag of ketchup. Under half an hour, and cheap enough to make a pile of them.
Melt the butter in a pan over medium heat. Cook the onion for 2-3 minutes until soft, then add the mushrooms. Cook 8-12 minutes, stirring, until all their water has cooked off and they start to brown. Season with salt, pepper and marjoram; cool slightly.
Heat the oven to 200°C (400°F). Halve the baguettes lengthwise. For a crisper base, pre-toast them bare for 3-4 minutes.
Spread the mushroom mixture over each baguette half and top generously with the grated cheese.
Bake for 8-10 minutes until the cheese is melted and bubbling and the bread is crisp and golden. For extra-bubbly, spotty cheese, finish 1-2 minutes under the grill.
Drizzle over a zig-zag of ketchup and scatter with chopped chives. Serve hot.
Notes:
Cook the mushroom water off completely before assembling - wet mushrooms make a soggy zapiekanka.Day-old baguette holds up far better than soft fresh bread.Grill at the end for that authentic street-stall bubbly, browned cheese.The ketchup drizzle is traditional, not optional - the sweet-tangy finish is the whole point.