Preheat oven to 400°F. Put mushrooms, ½ tsp. garlic, 1 tsp. salt, and rosemary in a bowl. Drizzle with olive oil and toss to coat; spread evenly onto a baking sheet. Roast mushrooms until golden brown, 20 to 25 minutes.

Melt butter in a large saucepan over medium heat. Add onions, thyme, and remaining 1 tsp. garlic; cook, stirring occasionally, until onions are softened, 7 to 10 minutes. Add sherry; cook, stirring occasionally, until almost evaporated, 1 to 2 minutes.

Reserve 1 cup roasted mushrooms. Purée remaining roasted mushrooms, onion mixture, broth, milk, and heavy cream in a blender until smooth, 1 to 2 minutes. (Vent blender to prevent steam buildup or use an immersion blender.) Return soup to saucepan; stir in pepper and remaining ½ tsp. salt. Cook soup over low heat, stirring frequently until soup just simmers, 5 to 10 minutes. Serve topped with reserved roasted

mushrooms, additional thyme, and, If desired, a drizzle of olive oil and more pepper.