Step 1

Arrange a rack in the middle of the oven and heat to 375°F. Place the cremini mushrooms on a rimmed baking sheet. Drizzle with the butter, sprinkle with the salt, and toss to combine. Spread into a single layer. Roast until mushrooms are tender, beginning to brown, and most of the liquid has evaporated, 15 to 17 minutes.

Step 2

Transfer the roasted mushrooms and their liquid to a 3 1/2- to 4-quart slow cooker. Add the onion, garlic, broth, sherry, porcini mushrooms, pepper, thyme, and tarragon, and stir to combine. Cover and cook on the LOW setting until the soup is fragrant and the dried mushrooms are very tender, 3 to 4 hours.

Step 3

Use an immersion blender, or transfer the soup in batches to a blender, and blend to desired consistency. Return to the pot if needed and cook on LOW for up to 2 additional hours. Before serving, stir in the cream and lemon juice. Ladle into bowls and garnish with an extra swirl of heavy cream and fresh chopped thyme or tarragon.