First make the hollandaise sauce. Put the vinegar, water, shallots, herbs and peppercorns in a saucepan and reduce the liquid to about a dessertspoonful. Strain it and put to one aside.
Melt the butter and simmer gently it for 5–10 minutes until it begins to separate. Remove it from heat, leave to cool a little, then pour off the pure butter where it has separated from the whey and discard the whey. This helps to keep the sauce thick.
Put the egg yolks into a small stainless-steel bowl with half of the vinegar reduction, and whisk over a pan of gently simmering water until the mixture begins to thicken and become frothy. Slowly trickle in the butter, whisking continuously – an electric hand whisk will help. If the butter is added too quickly the sauce will separate.
When you have added two-thirds of the butter, taste the sauce and add a little more or all of the reduction and season with salt and pepper. Then add the rest of the butter. Season again with salt and pepper if necessary, lay some clingfilm directly on the sauce to stop it forming a skin and leave in a warm (not hot) place until needed. The sauce can be reheated briefly over a bowl of hot water and lightly whisked again, but try to avoid if possible.
To serve, lightly toast the muffins and soft-poach the eggs. Reheat the spinach, lay it on the muffins, then place a slice of smoked salmon on the muffin with the poached egg on top and coat it with a couple of generous spoonfuls of the Hollandaise sauce and as much of the salmon eggs as you want.