Add 3 tablespoons of butter to a medium pan over medium heat.
Once the butter is bubbly and melted, add the finely chopped mushrooms and the diced onion. Season the mushroom and onion mixture with a pinch of salt and ground black pepper. Cook, stirring frequently for 7 to 10 minutes or until the onion is soft and the mushrooms are tender. Remove the pan from the heat.
Slice your bread roll all the way and place both halves on a cutting board or plate, cut side up.
Preheat your oven to 375 degrees F (190 C).
Cover each side of the cut bread with a generous layer of the mushroom and onion mix and then top that with shredded cheese. Place on a sheet pan that has been lined with aluminum foil.
Once the oven is preheated add the sheet pan and bake for 10 minutes or until the bread is crunchier and the cheese has fully melted.
Top the zapiekanka with crunchy fried onions, parsley, and a zig-zag of ketchup.
Serve open-faced like a French bread pizza and enjoy.