For the hollandaise, put the egg yolk in a heatproof bowl set over a pan of barely simmering water. Whisk for 1 minute until pale and thick. Remove from the heat and whisk in the lemon juice, 1 tsp water and softened butter.
Gradually add the melted butter, whisking until the mixture is a pouring consistency. Add the mustard, season and keep warm.
Wilt the spinach in a pan with 1 tbsp water. Drain well and season.
Bring a pan of water to the boil, add the vinegar and reduce to a simmer. Use a whisk to create a vortex, then crack the eggs into the centre. Cook for 2-3 minutes until the whites are set but the yolks are still runny.
Divide the toast between plates. Top with the spinach and poached eggs. Spoon over the hollandaise, season with black pepper and serve.