Place grated potatoes in a large bowl, season with salt and stand for 2-3 minutes. Place mixture into a sieve and squeeze out any excess moisture.
Heat a frying pan on medium high heat, add 2 teaspoons oil and 1 teaspoon butter. Place a teaspoonful of the potato mixture into a hot pan; flatten with a spatula to form a small disc. Cook 6 rosti at a time until golden brown: turn and cook the other side.
Drain on paper towel and cover to keep warm. Repeat with remaining potato mixture, wiping the pan clean with paper towel between each batch.
Mash John West Scottish Sardines using a little of the tomato sauce to make a soft consistency. Add chopped pine nuts and combine. Place spoonfuls of sardine mix on top of cooled rosti and serve garnished with a baby spinach leaf.