Grate the sweet potato using the larger holes of the grater. Mix with 1 whole egg, half a teaspoon of sea salt and a pinch of cracked black pepper.
Heat a large frying pan with a tablespoon or two of ghee or oil. Transfer the potato mix to the middle of the pan, spread and flatten using a spatula until the layer is nice and thin, like a large pancake.
Turn the heat down to medium and fry the roast, covered with a lid, for 3-4 minutes. To flip the pancake without breaking it, put a large plate face down on top of the pancake and turn the frying pan upside down. The pancake will fall on your plate, cooked side up. Add another 1/2 teaspoon of fat or oil to the frying pan and once melted, slide the pancake back into the pan. Cook uncovered on medium heat for 4-5 minutes, until golden brown on the bottom as well.
While the rosti is cooking, combine the cucumber ribbons with onions and lemon juice.
To assemble, carefully transfer the rosti pancake to a large serving plate. Scatter the slices of smoked salmon and top with cucumber strips and red onion strips on top. Drizzle with some extra lemon juice and avocado or olive oil, finish up with a sprinkle of black pepper. Add a dollop of mayo on top. Fresh dill would be the ideal herb to garnish with.