Heat the oven to 400 degrees. On a sheet pan, toss together the mushrooms, shallots, thyme, oil and soy sauce until well combined. Sprinkle with salt and pepper and spread into an even layer.

Roast until the mushrooms are pooling in liquid, about 15 minutes. Carefully pour the liquid into a small bowl and reserve. Return the sheet pan to the oven and roast, stirring occasionally, until browned, another 25 to 30 minutes.

Discard the thyme sprigs and set aside ½ cup of the roasted mushrooms. If using an immersion blender, transfer the remaining mushrooms to a pot, or transfer to a regular blender. To the sheet pan, add some of the water (about ½ cup but no need to measure) and scrape off the stuck-on browned bits. Pour the mixture into the pot or blender, along with the remaining water, reserved mushroom liquid and vinegar. Blend until smooth.

Warm the soup on the stove over medium-high. Turn off the heat and stir in the cream. Taste; if the flavor is bland, add salt and pepper; repeat until the mushroom flavor is pronounced. Serve topped with a drizzle of cream, the reserved mushrooms and thyme leaves.