Preheat the oven to 180°C (356°F) fan.
Remove the base of the springform pan.
Place the top of the round pan on a piece of parchment paper.
Brush the parchment paper lining inside the round pan with 1 tablespoon of melted butter.
Mash 6-8 cooked potatoes flat in the round pan using a burger press.
Top the remaining melted butter over the smashed potatoes.
Use a pastry brush or the back of a spoon to spread it all over the potatoes.
Sprinkle with sea salt, black pepper, and oregano.
Bake for 20-25 minutes, until the top starts to brown and the crust is golden.
Remove the cake pan frame.
Top the warm potato disc with tomato sauce, chopped red onion, mozzarella, and pepperoni.
Bake for 10 minutes, then turn up the heat to 220°C (428°F) fan.
Bake for an additional 8-10 minutes until the cheese is bubbly and golden.
Carefully loosen the pizza from the parchment paper with a spatula.
Top with fresh basil.