Cut mushrooms into bite-size pieces. In a large saucepan over medium heat, melt butter with a drizzle of olive oil. Add mushrooms and salt. The pot will look crowded, nearly three-quarters full, but moisture release is expected. Mushrooms are water-dense and will reduce significantly.
Cook over medium-high heat until liquid releases and mushrooms soften. Once reduced to roughly a quarter of their original volume, add diced white onion and roasted garlic. Onion powder and garlic powder may be used in a pinch. Stir until fragrant and golden brown.
Lower to medium-low heat. Add thyme and cooking wine, allowing alcohol to evaporate fully. Pour in vegetable stock until mushrooms are just submerged. Bring gently to temperature.
In a measuring cup, whisk cornstarch with cold stock to form a cornstarch slurry. When the soup reaches roughly 150°F, stir in the slurry. Simmer for 20 minutes, stirring occasionally, until thickened.
Add heavy cream and simmer for five additional minutes. Adjust seasoning with black pepper. Blend partially using an immersion blender or food processor for a smooth yet textured cream soup. Taste. Adjust salt. Garnish with fresh parsley.
Serve hot with crusty bread or gluten-free garlic bread.