On a large rimmed baking sheet, toss together cremini mushrooms, shiitake mushrooms, onion, garlic, thyme, and olive oil.
Bake until mushrooms are browned and tender, about 30 minutes.
In a large bowl, combine porcini mushrooms and 2 cups boiling water; cover and let stand for 20 minutes.
In a large Dutch oven, melt butter over medium heat; add flour and cook, stirring constantly, for 2 minutes. Gradually add broth; bring to a boil, reduce heat, and simmer until mixture is slightly thickened. Stir in roasted mushroom mixture and porcini mushrooms with soaking liquid.
In the container of a blender, process mixture in batches until smooth.
Return soup to pot. Stir in cream and Madeira. Cook, stirring occasionally, until thoroughly heated. Stir in sour cream. Season with salt and pepper to taste. Garnish servings with toasted bread crumbs, sautéed mushrooms, thyme leaves, and cheese, if desired. Refrigerate for up to 3 days.
Recipe by Victoria at https://victoriamag.com/roasted-mushroom-and-onion-soup-recipe/