Step 1

Roughly chop mushrooms. Add oil and butter to a large, heavy pot and cook over medium-high until butter melts. Add mushrooms, salt, and pepper; cook, stirring often, until mushrooms are deeply browned and most of the moisture has evaporated, 8 to 10 minutes.

Step 2

Add shallots and scallions; cook, stirring often, until softened, about 4 minutes. Add flour and stir to combine. Add stock and milk, stirring and scraping to loosen any browned bits from bottom of pot. Let mixture come to a low boil and cook, stirring occasionally, until broth thickens, about 5 minutes. Remove from heat. Transfer half of mixture to a blender. Secure lid on blender and remove center piece to allow steam to escape. Place a clean towel over opening and process until almost smooth; return to pot. (Or use an immersion blender to pulse the soup until somewhat blended but some whole pieces of mushroom are still visible.) Stir in parsley and tarragon; serve hot.