Scoop the butter into a frying pan, and set it over low heat to start melting.
Meanwhile, peel and chop the onion relatively finely.
Crank up the heat a bit on the frying pan, and add the onion in.
While the onion is sautéing, wash the spinach, shake off excess water, then chop it up roughly.
When the onion is starting to get translucent, add the spinach and raise the heat on medium. Continue sautéing until the spinach first wilts and gives up its water, and then the water evaporates away. Spread the spinach out evenly in the pan, then make six nests for the eggs. Crack 1 egg into each nest. Cook until the egg whites are covered. (You may cover the pan if you wish the yolks to cook better and faster.)
Optional: when you crack the eggs in, sprinkle each with a bit of dried red pepper flake, or chilli powder.