Heat oven to 425°F. Toss mushrooms with olive oil, 1/2 teaspoon salt, and pepper on a large rimmed sheet. Roast 25 to 30 minutes, turning once, until browned and moisture mostly evaporates.
Melt butter in a soup pot over medium. Cook shallots 5 minutes, garlic 30 seconds; add sherry and simmer 2 minutes.
Scrape mushrooms and juices into pot. Add broth, thyme, and remaining salt; simmer 10 minutes.
Carefully puree half the soup, including mushrooms, in a heat-safe container. Return to pot, leaving other mushrooms intact.
Stir in crème fraîche over low until melted and steaming, about 3 minutes. Divide among 4 bowls; avoid a hard boil.