Spring greens soup

3 cups lightly packed baby spinach;

4 cups lightly packed watercress, arugula, dandelion greens, and/or nettles;

1 cup lightly packed sorrel;

3 tablespoons butter;

1 cup chopped leeks, scallions, or spring onions (white and tender green parts);

4 cups chicken stock;

1 teaspoon kosher salt, plus more to taste;

8 ounces starchy potatoes, peeled and diced (such as russets);

1 cup heavy cream;

1⁄4 teaspoon ground white pepper;

3 large egg yolks (optional).

Fill a large bowl with ice water. Bring a large saucepan of water to a boil. Add 1⁄2 teaspoon kosher salt per cup of water. Place all the greens in a strainer and lower into the water for 1 minute. Transfer into the ice water to stop the cooking and set the color. Drain the greens, squeeze dry, coarsely chop, and set aside. There will now be only a paltry handful of greens, which is the way of cooked greens.
Melt the butter in a large saucepan over medium heat. Add the leeks and a pinch of salt and cook, stirring often, until softened, about 10 minutes. Do not let them brown.
Stir in the stock, salt, and potatoes. Bring to a boil, reduce the heat, and simmer until the potatoes are very tender and starting to fall apart, about 25 minutes. Stir in the greens.
Puree in a blender (working in batches to not fill the blender more than half full) and return to the pot, or puree the soup directly in the pot with an immersion blender. Be patient; it takes a while to puree cooked greens.
Bring the soup to a gentle simmer and stir in the cream. Season with salt and the white pepper.
If you are adding the yolks, whisk them in a medium bowl until smooth. While whisking constantly, slowly pour in a ladleful of hot soup to temper the yolks. Stir the yolks into the soup and heat through. Do not let the soup boil or the yolks might curdle. Serve warm.