01 - Melt butter in a large pot over medium heat. Add onions and shallots; sauté for 3-4 minutes until softened and translucent.
02 - Stir in minced garlic and cook for 1 minute until fragrant, being careful not to brown it.
03 - Add sliced mushrooms and sauté for 7-8 minutes until mushrooms are golden brown and their moisture has completely evaporated.
04 - Add thyme sprigs and bay leaf to the pot, stirring to combine with the mushroom mixture.
05 - Pour in vegetable broth and bring to a gentle boil. Reduce heat to low and simmer uncovered for 15 minutes to develop flavors.
06 - Discard the thyme sprigs and bay leaf from the soup.
07 - Use an immersion blender to puree the soup until completely smooth and velvety. Alternatively, blend in batches using a countertop blender, then return to pot.
08 - Stir in heavy cream, black pepper, and salt. Simmer gently for 5 more minutes; do not allow the soup to come to a boil.
09 - Taste the soup and adjust salt and pepper as needed to achieve proper seasoning balance.
10 - Serve hot in warmed bowls, garnished with fresh thyme leaves and a drizzle of olive oil. Accompany with crusty bread if desired.