Melt the butter in a pan, add the onion and spinach, sauté for approx. 5 mins. until all the liquid has evaporated. Season the spinach, keep warm.
Bring the water and vinegar to the boil in a pan, reduce the heat. Crack the eggs individually into a cup, carefully slide into the gently boiling water, poach for approx. 4 mins. at just below boiling point, remove with a slotted spoon, keep warm on a plate.
Heat the Hollandaise sauce in a small pan. Lightly toast the slices of bread in a toaster and arrange on plates. Top with the spinach, eggs and sauce, season and scatter over the chives.