Start the hollandaise: simmer the lemon juice, vinegar, bay leaf and peppercorns in a small pan until reduced by half. Strain and leave to cool. You can do this the night before and leave in the fridge.
Put the fish in one layer in a shallow pan. Cover with cold water and add the onion, bay leaf and peppercorns. Cover with a tight-fitting lid and bring to a gentle simmer. Turn off the heat and leave the fish for 8 minutes to stand while you make the hollandaise.
Put the yolks in a blender and whiz for a few seconds. With the blender running, add the lemon juice mixture you prepared earlier. Then slowly pour in the melted butter. The mixture will thicken to the consistency of single cream. Check the seasoning and keep warm.
Rinse the spinach and put in a large pan with a generous pinch of salt and a grating of nutmeg (the water clinging to the leaves is sufficient so don’t add any more). Cover with a tight-fitting lid and cook over a medium heat, stirring occasionally, until the spinach has wilted. Drain and squeeze out excess water. Roughly chop and keep warm.
To assemble the dish, put a slice of buttered toast on each plate. Top with a portion of spinach. Remove the fish from the pan with a fish slice, patting away any excess liquid with a piece of kitchen towel and put on top of the spinach. Finish with a poached egg and a generous spoonful or two of hollandaise sauce.