Preheat the oven to 200 C (400F). Line a baking sheet with parchment paper and place mushrooms in a single layer on the baking sheet.
In a small bowl, stir together the olive oil, half of the minced garlic, 4 sprigs of thyme, and some salt and pepper. Drizzle this mixture onto the mushrooms and toss to combine.
Roast in the hot oven for 20-30 minutes or until browned and tender, stirring midway through. Let mushrooms cool, then chop coarsely.
Melt the butter in a saucepan over medium heat and add the diced onion. Cook for 12-15 minutes, stirring until onion is golden and caramelized. Add the remaining garlic and cook for 1 additional minute.
Whisk in the flour until lightly browned, about 1 minute. Stir in water.
Add the chicken stock, bay leaves, and the rest of the thyme. Season with salt, pepper and nutmeg. Bring to a boil then reduce heat to a simmer and simmer for 15 minutes until slightly reduced.
Stir in the mushrooms and simmer for another 10 minutes until thickened. Remove from heat and stir in the heavy cream.
You can either leave the soup chunk, or use an immersion blender and puree to desired consistency.