Spinach wilted, blended and stirred into a butter and flour roux loosened with milk. Twenty minutes, five servings, and the roux is what makes it a cream rather than a green broth.
Spinach is almost all water. Five hundred grams looks like an enormous amount raw and collapses to a couple of handfuls in five minutes, which is why it needs only a cup of water to cook rather than the three the recipe used to call for. All of that water ended up in the finished cream, and it is the reason this soup had a reputation for coming out thin. Cook the flour in the butter for a minute before the milk goes in, and add the milk warm and gradually, stirring the whole time. That is where lumps form and nowhere else. Twelve minutes over low heat at the end is what cooks the flour out and lets the cream thicken properly. It thickens further as it cools, so stop while it still moves easily in the pot.