Preheat oven to 350°F (180°C).
Wash fingerlings and place into a pot of cold, salted water, then bring to a boil. Reduce heat and cook until fork tender. Remove from heat and drain. Transfer to a bowl and allow to cool, then slice thinly. Add salt and oregano and toss to coat.
In a medium pan, heat butter on low heat. Add sliced onions and cook for about 1 hour, until nicely caramelized.
Add pancetta and salami to a frying pan and cook for 10 minutes over medium heat, until crispy. Remove from pan and set on paper towel to absorb excess grease.
Place flatbread on a baking dish and spread evenly with pizza sauce. Top with cheese and both meats. Place potatoes evenly on flatbread and top with caramelized onions. Bake for 10-15 minutes, then remove from oven and slice. Enjoy!
Optional: Add fresh arugula before serving.