Cook spinach in a large pot with 1/4 inch water until steam has collapsed leaves, 8 to 10 minutes. Drain, and squeeze out liquid. Coarsely chop.
In a medium skillet over medium heat, melt butter. Whisk in flour, 1 teaspoon salt, and some pepper. Stir for 1 minute. Whisk in milk, and simmer until slightly thickened, about 1 minute. Fold in chopped spinach.