I started playing with this soup late last year, when my husband was away at some cocktail event and I had to fend for myself for dinner. I wanted onion soup, but I didn’t want to spend a lot of time on my standard version. So I looked in the fridge to figure out a plan B, and there they were – two fat leeks, just waiting to be put to good use. I decided to roast them to coax out their sweetness, then I pureed them with a bit of seasoned stock to create a creamy texture without a lot of added fat (though I did add a little butter at the end in the form of a beurre manie, to further thicken the soup). The result was a delicate, velvety soup that was surprisingly elegant for a something that came together so easily. —lastnightsdinner
Trim the dark green tops and the root ends of the leeks, leaving only the white and pale green parts. Slice the leeks in half lengthwise, then slice again crosswise into about 2 to 3 inch long sections. Swirl the leeks around in a bowl or basin of water to remove any dirt or grit between the layers (it’s okay if some of the layers separate), then rinse them well under running water. Dry thoroughly with a kitchen towel.
Place the cleaned leeks on a baking sheet and sprinkle with salt. Drizzle the olive oil over them and place into the oven to roast until soft, about 20 minutes.
Warm the stock in a heavy-bottomed pot until simmering. Whisk in the dry mustard, breaking up any clumps.
Mash the flour and butter together to form a paste and set aside.
Place the roasted leeks in a blender, scraping up all of their accumulated juices to add as well. Add a cup of the hot stock and puree until smooth. Pour the leek puree into the simmering stock, add the thyme, and increase the heat until the soup begins to bubble. Whisk in the flour-butter paste until well incorporated, then reduce the heat, cover, and simmer for another 10-15 minutes. Taste and adjust seasoning, if needed, then ladle into warmed bowls.