Cut a small slit in the center of each bologna slice to help prevent it from bubbling or curling during frying.
Place a skillet over medium heat and add half of the butter, along with the vegetable oil if using.
Add the bologna slices to the hot pan and cook for 2–3 minutes per side, until deeply browned and slightly crisp around the edges.
Place the cheese on top of the bologna during the final 30 seconds of cooking, allowing it to melt.
In the same skillet, melt the remaining butter and toast both bread slices until golden on one side.
Spread mustard, mayonnaise, or your preferred condiments over the toasted bread.
Place the fried bologna and melted cheese on one slice of bread. Add black pepper and any optional toppings you like.
Top with the second slice of bread, close the sandwich, and serve immediately while hot and crispy.
