Combine the grated sweet potato, egg white, tapioca starch in a bowl, and season to taste with salt and pepper.
Heat some coconut oil in a skillet placed over a medium-high heat.
Drop a spoonful of the sweet potato mixture into the skillet and flatten with a spatula to form a small pancake. Cook for about 5 minutes on each side.
Repeat step 3 with the remaining sweet potato mixture.
In a bowl, combine all the ingredients for the mayonnaise, mix well, and season to taste.
Top each rosti with flavored mayonnaise, smoked salmon, and green onions.