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Heat the oil and butter in a frying pan over a medium heat until foaming, then fry the mushrooms for 10 mins until just golden brown. Add the garlic and spinach and fry for 5 mins more, then season with salt and black pepper and set aside.
To make the hollandaise, tip the vinegar, peppercorns, bay leaf and 40ml water into a small pan. Simmer over a low heat until the liquid has reduced by two-thirds. Remove from the heat and set aside.
Fill a large pan a third full with water and bring to a gentle simmer. Whisk the egg yolks with 1 tbsp of the vinegar mixture in a heatproof bowl for 3-5 mins until just aerated. Set the bowl over the simmering water and slowly drizzle in the melted butter, whisking until all the butter has been incorporated and the mixture has emulsified. Remove from the heat and taste for seasoning. Add a little more of the vinegar mixture or salt and pepper, if you like.
Bring a pan of lightly salted water to the boil, then reduce the heat to a simmer. Wipe a little vinegar around a small ramekin using kitchen paper, then crack in one of the whole eggs. Swirl the simmering water using a wooden spoon and wait for the vortex to begin to subside. When it does, gently tip the egg into the middle and cook for 3 mins. Scoop out onto a plate using a slotted spoon, then repeat with the remaining eggs. Reheat the mushrooms over a low heat, then pile these on top of the toast. Top with the poached eggs, hollandaise and chives or tarragon.