1 1/2 tablespoon all-purpose flour (sifted to remove any lumps)
1 cup beef stock
2 tablespoons butter
Start by bringing a large pot of salted water to boil (about 1 1/2 tsp per quart of water). Add the spinach and boil for 2 minutes or until the spinach is almost tender. Test it by eating a piece. Drain. Rinse in cold water and squeeze as much water out of the spinach as you can, doing so in small amounts at a time. Chop the spinach with a big knife on a chopping board or if you want a fine puree, put it through a blender.
In a large pan over high heat, melt butter until it bubbles and add spinach. Stir the spinach until the humidity has evaporated, lower heat to medium, sprinkle on the flour and stir for 2 minutes. Remove from heat and stir 2/3 of the stock (or cream by spoonfuls). Bring to simmer, cover, and cook very slowly for about 15 minutes. Stir frequently to prevent the spinach from sticking to the bottom of the pan, and add more liquid if the spinach becomes too dry. Season with salt and pepper, and serve.