Put the grated potato and onion together into a clean tea tower and squeeze out all of the liquid.
Put the dried potato and onion into a mixing bowl with the chill flakes and melted plant butter. Stir to combine, then season well with salt and pepper.
Place a non-stick medium frying pan (about 23cm diameter) over a medium heat. Once it's hot, fully fill the pan with a thick layer of potato mixture. Don't press the potato too much into the pan, you still need there to be little air pockets so that the steam can cook the rösti all the way through (if it's too compressed it won't cook in the centre). Leave to cook for 15-20 minutes, not turning up the heat too high - think low and slow. You can tell when the edges start to go translucent/slightly brown that the rösti is ready to flip.
To do this, I recommend placing a chopping board or large plate on top of the frying pan, inverting it so that the rösti lands on the board or plate, then carefully sliding the rösti back into the frying pan, uncooked side down, to continue. Cook for another 10 minutes, until golden and crisp on the underside and cooked all the way through.
Transfer the rösti to a chopping board and slice it into wedges. Put a blob of crème fraîche on each slice, top with a few pieces of caviar and sprinkle over some flaky salt, chives and dill, to finish.