One 12-ounce head romaine, cored and cut into chunks, plus shredded romaine, for serving
Melt the butter in a large saucepan over medium heat. Add the flour, onion, 1 3/4 teaspoons salt and a couple turns of pepper and cook until the onion is softened, about 6 minutes. Whisk in the milk and simmer for 10 minutes. Stir in the lettuce and cook, turning with tongs, until bright green, about 1 minute. Working in batches, blend the soup until smooth; return it to the pot to heat through and thin with milk or water as desired. Serve topped with shredded romaine.
Disclaimer: When blending hot liquid, first let it cool for 5 minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters and pulse until smooth.
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