Gently melt the butter and let it cool a bit. Whisk the egg yolk with the lemon juice and a spoonful of water in a bain-marie until light and thick, then gradually whisk in the melted butter in a thin stream until a smooth hollandaise forms. Season with salt and keep warm.
Bring a pan of water with vinegar to a boil. Break the eggs one by one into a cup and gently slide them into the gently simmering water. Poach for three minutes until the whites are set and the yolk is still runny.
Meanwhile, heat the olive oil in a frying pan and let the spinach wilt in half a minute with some salt and pepper. Distribute the spinach over a plate, place the smoked chicken on top and slide the poached eggs on top. Drizzle the hollandaise over and grind black pepper over it. A Sunday breakfast that feels indulgent but contains virtually no carbohydrates.