How to Make It
Remove lid from a 6-quart Instant Pot®. Press [Sauté], and use [Adjust] to select "More" mode. When the word "Hot" appears, swirl in 2 teaspoons of the oil. Add leek; cook, stirring constantly, 1 minute. Add 1/8 teaspoon of the salt. Cook 5 minutes or until leeks are softened, stirring occasionally. Add cauliflower, 1 cup and 6 tablespoons of the stock, 3/4 cup water, and thyme. Turn cooker off.
Close and lock the lid of the Instant Pot®. Turn the steam release handle to "Sealing" position. Press [Manual]; select "High Pressure," and use [-] or [+] to choose 4 minutes pressure cooking time. When time is up, turn cooker off. Open the cooker using Quick Pressure Release (page 4). Remove inner pot from cooker.
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 inner pot. Stir in remaining 1/4 teaspoon salt, milk, butter, and pepper. Return inner pot to cooker. Press [Keep Warm].
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.
Ladle soup evenly into 4 bowls. Top each serving with mushroom mixture. Sprinkle evenly with parsley.