Make this right before serving, or it'll lose its color.
Place wet, washed spinach in a saucepan, add salt, and cook until just
slightly pooped and very deep green. Puree the cooked spinach in its
own liquid, in a blender.
Melt butter. Whisk in flour. Cook, whisking 2-3 minutes. Add spinach
puree and cook 5 minutes, stirring frequently. Add seasonings (and
onions and/or cheese). Just before serving whisk in the yogurt and
sour cream.
Good on grains, noodles (add some parmesan); baked potatoes, poached
eggs.
