Step 1

Heat 3 Tbsp. oil in a large heavy pot over medium-high. Cook half of mushrooms, undisturbed, until browned underneath, 3–4 minutes. Toss and continue to cook, tossing occasionally, until browned on both sides and tender, 4–5 minutes more. Season with salt and pepper and transfer to a medium bowl. Repeat with 3 Tbsp. oil, remaining mushrooms, and salt and pepper. Set aside 1 cup mushrooms; transfer remaining mushrooms to bowl.

Step 2

Heat same pot over medium. Cook shallots, leek, butter, and half of garlic, stirring occasionally, until vegetables are softened but not browned, 4–6 minutes. Add wine and cook, stirring, until liquid is evaporated, 2–3 minutes. Stir in flour and cook until vegetables are coated, about 1 minute. Whisk in stock, cream, and milk and bring to a boil. Add bay leaf, 3 thyme sprigs, and mushrooms (not including the 1 cup) and any accumulated juices. Simmer, uncovered, until soup thickens to a heavy cream consistency, 25–35 minutes. Remove thyme and bay leaf, season with salt and pepper, and add reserved 1 cup mushrooms. Return to a simmer.

Step 3

Meanwhile, pulse bread in a food processor until coarse crumbs form; season with salt and pepper. Heat remaining ¼ cup oil in a large skillet over medium. Cook remaining 2 garlic cloves and 2 thyme sprigs, stirring occasionally, until garlic begins to brown, about 2 minutes. Add breadcrumbs and cook, stirring often, until golden brown and crisp, 7–10 minutes. Transfer breadcrumb mixture to paper towels. Remove thyme; season with salt.

Step 4

Divide soup among bowls, then top with toasted breadcrumbs and a few twists of pepper.