How to Make It
Melt 1/4 cup butter in a large skillet over medium-low. Add leeks, garlic, and salt; cook, stirring often, until leeks are tender, about 15 minutes. Transfer to a blender; add parsley, 1/2 cup dashi, and 1/2 cup water. Process until smooth. Pour through a fine wire-mesh strainer into a bowl; press with a rubber spatula. Discard solids.
Rinse blender, and wipe dry. Return leek mixture to blender, and remove center piece of blender lid. With blender running, gradually add 1/4 cup butter, 2 teaspoons at a time, processing until mixture is smooth and velvety, about 1 minute and 30 seconds. Transfer leek puree to a medium bowl; stir in up to 2 tablespoons water, 1 teaspoon at a time, as needed until mixture reaches desired consistency (consistency should be just thicker than heavy cream). Set aside.
Melt remaining 1/4 cup butter in a large skillet over medium. Add onion; cook, stirring often, until softened and beginning to brown, 8 to 10 minutes. Add mushrooms; cook, stirring occasionally, until tender, 6 to 8 minutes. Add peas and remaining 1/4 cup dashi; cook, stirring often, until liquid has completely cooked off, 1 to 2 minutes. Remove from heat. Stir in tarragon, lemon juice, mustard, and 1/4 cup leek puree. Season with salt.
Spoon 1 1/2 tablespoons leek puree onto each of 8 plates. Top each with 1/4 cup mushroom mixture and 1 buckwheat crêpe cracker. Garnish with miner’s lettuce and garlic flowers. Serve immediately.
Shiro dashi can be ordered from Nishiki Pacific.