Photograph: Jonathan Gregson
Melt 50g of butter in a frying pan, then cook the onions, thyme and a pinch of salt for 8-10 minutes until soft. Add the vinegar and bubble until syrupy. Cool, then mix with the crème fraîche.
For the pancakes, mix together the ricotta, egg yolks, flour and a pinch of salt. Whisk the egg whites until fluffy and fold into the pancake mixture.
Heat a tablespoon of butter in a nonstick frying pan and drop small spoonfuls of the pancake mixture into the pan. Flatten slightly and cook until golden, then flip over and cook the other side. Do this in batches until you have 24 pancakes. Allow to cool.
Slice the salmon into thin strips and roll tightly. Spread some crème fraîche mixture on to each pancake, then top with a salmon roll and a basil leaf.
Get ahead: make up to the end of step 3 the day before; chill.