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 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.

Polish sausage and onions: when the zapiekanka goes into the oven, slice your onion into rings, then cut each onion ring in half.  Place both the onions and sausages in a warm pan or griddle over medium-high heat and cook for 8 minutes, flipping the sausage over a couple of times to ensure proper browning. 

After 8 minutes of cooking the sausage, cut each sausage in half lengthwise and lay each piece flat on the hot pan to sear the inside a little.

Once the zapiekanka comes out of the oven, it's time to build the open-faced sandwich.

Sandwich assembly: place 4 sausage pieces on top of the cheese on each zapiekanka. Top each one with half of the cooked onions and then top the onions with 4 or 5 sport peppers or 10 slices of pickled jalapenos.

Squirt a squiggly line of mustard on top of everything and then serve the zapiekanka open-faced like a French bread pizza.