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Rinse the mushrooms with a mushroom brush or paper kitchen towel. Divide the mushrooms into slices. Peel and finely chop the onion or shallot and divide the baguette into halves.
Pre-heat the oven to 180 degrees Celsius or 350 degrees Fahrenheit. Heat a knob of butter in a skillet and cook the mushrooms along with the finely chopped onion for about 5 minutes.
Divide the mushrooms and onion over the baguettes. Divide about a handful of grated cheese over the sandwiches and bake them in the oven for about 5 minutes.
Finish the zapiekanka with tomato ketchup. Finely chop the chives and garnish the sandwiches with chives. Serve immediately and enjoy!