Set up your Big Green Egg for a direct cook at 220°C.
Pop either a cast iron skillet or one of the Solidteknics Bigga pans, into the Egg to warm up.
Part boil your potatoes for about 5 minutes, you're just trying to warm them through and get the starch within them working.
Add 2-3 tablespoons of olive oil to the skillet and add the mushrooms and onions to fry.
Sautee the mix for about 5 minutes.
Use a box grater to grate your potatoes. The gratings will be sticky from the starch.
Portion your potatoes into rosties.
Add some more oil to you pan and then the rosties. Cook them for about minutes a side.
Continue to mix your mushrooms and onions, adding the salt and pepper.
When the rosties are golden brown on each side and the mushroom and onions soft, add the rosties to a plate and top them with the mix.