For the salmon, heat a dash of olive oil in a frying pan, season the salmon with salt and freshly ground black pepper and fry on both sides for 2-3 minutes, until browned on the outside and lightly cooked in the middle. Remove from the heat and set aside.
For the leek mash, place the chopped potato into a pot of boiling salted water and cook for 7-8 minutes until tender. Gently fry the baby leek in a little butter until soft and translucent. When cooked, drain the potatoes and then pass through a ricer or mash with a potato masher. Add the butter and cream, season with salt and freshly ground black pepper and mash until smooth. Stir in the fried leek.
For the hollandaise, whisk the white wine vinegar, egg yolks and mustard together in a bowl until well combined. Place the bowl over a pan of boiling water, making sure the bottom of the bowl does not touch the water, and whisk until thickened slightly.
Pour in the melted butter a little at a time, whisking continuously until all the butter has been added and the mixture has emulsified. Add the lemon juice, salt and freshly ground black pepper to taste.
To serve, pile the mash onto a serving plate, top with the salmon and drizzle over the hollandaise.