1. Heat a large saucepan over high heat. Add 2 teaspoons oil to pan; swirl to coat. Add leek; sauté 1 minute. Add 1/8 teaspoon salt. Cover, reduce heat to low, and cook 5 minutes or until leeks are softened, stirring occasionally. Add cauliflower, 1 cup and 6 tablespoons stock, 3/4 cup water, and thyme. Bring to a boil; cover, reduce heat, and simmer 7 minutes or until cauliflower is very tender. Place cauliflower mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Return to saucepan. Stir in remaining 1/4 teaspoon salt, milk, butter, and pepper. Keep warm.
2. Thinly slice mushroom caps. Heat a large skillet over medium-high heat. Add remaining 2 teaspoons oil to pan, and swirl to coat. Add mushrooms; sauté 6 minutes or until browned. Add remaining 2 tablespoons stock, Worcestershire sauce, and sherry vinegar. Cook 1 minute or until liquid is reduced and syrupy.
3. Spoon about 1 cup soup into each of 4 bowls. Top each serving with about 2 tablespoons mushroom mixture. Sprinkle evenly with parsley.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.