Dissolve the yeast in lukewarm water and add sugar. Combine the flour and salt in a bowl. Make a well in the flour and add the yeast water. Knead the dough until silky whilst adding the olive oil. Shape into a ball, cover and leave in a warm place for 30 minutes to rise.
Then divide into four balls and roll out wafer thin on a floured surface. Wash the spring onions and cut into thin rings. Grate the Alpine cheese. Place the four tartes on a sheet of baking paper and spread evenly with crème fraiche. Add the spring onions and smoked salmon. Season with salt and pepper and sprinkle with Alpine cheese.
Preheat the oven to 220 °C with top and bottom heat and then bake for approximately 10 minutes. Sprinkle with lemon crema and serve warm.