Put the potatoes in a pot with the water. Bring the water to a boil and cook the potatoes at a strong simmer for 20 minutes or until tender. Drain the water from the pot and let the potatoes cool for only a few minutes before you begin to pull the skins from the potatoes. Discard the potato skins and, while still hot, pass the potatoes through a ricer into a bowl.
To the riced potatoes add the spinach and mix well.
Add the eggs, one at a time, and incorporate well.
Add the flour in two stages so that only the amount that is necessary will be used to bind the potato. Add the second half of the flour (and have some additional flour available if necessary) and mix well to form a dough.
Divide the dough in half, and on a floured work surface, roll the first half of dough into a log 1-inch thick. Cut the log into 1/2-inch thick round pieces.
Lay the pieces out, and the back of a fork or your thumb, make an indentation on one side.
Set aside and refrigerate until needed.
Cook as you would pasta in several quarts of boiling water for 10 to 15 seconds. They cook very quickly and are cooked when they float to the surface.
Heat the butter in a saucepan, add the poppy seeds and pour over the gnocchi in a large serving bowl.