Pre-heat the oven to 200C and get a 12 hole muffin tin out
Slice the potatoes using a mandolin (highly recommended) or a very sharp knife - you want them in very fine slices, about 1-2mm only
In a large mixing bowl, stir together mozzarella, cheddar, parmesan, oregano, thyme, garlic, butter and passata - make sure it's combined really well. Season with salt and pepper
Add the potato slices and pepperoni to the butter and passata mixture and stir well to coat all of the potato with the sauce
Using your hands, prepare the potato and pepperoni slices into 12 small stacks, placing them into each muffin hole as you go - make sure you distribute the cheese and pepperoni evenly throughout. You want the stacks to be at the top of the muffin holes
Cover with tin foil, place on a an additional baking sheet and place in the oven for 30 minutes
Remove the tin foil and return to the oven for a further 25 minutes - or until golden brown and bubbling
Allow to rest for 10 minutes minimum to set and then serve your pepperoni pizza potato stackers!