Clean the mushrooms with a damp cloth and slice them evenly, about quarter-inch thick for consistent cooking.

Heat a large saucepan or Dutch oven over medium-high heat and add butter and olive oil, allowing butter to melt completely.

Add sliced mushrooms in a single layer without overcrowding, cooking undisturbed for 3-4 minutes until golden brown on one side.

Stir mushrooms and continue cooking for another 5-6 minutes until deeply caramelized and reduced in volume by half.

Add diced onions to the pan and cook for 4-5 minutes until softened and translucent, stirring occasionally.

Stir in minced garlic and cook for 1 minute until fragrant but not browned, stirring constantly.

Sprinkle flour over the mushroom mixture and stir well, cooking for 2-3 minutes to eliminate raw flour taste.

Gradually pour in chicken or vegetable broth while stirring constantly to prevent lumps from forming.

Bring the soup to a gentle simmer and cook for 10-12 minutes, stirring occasionally, until slightly thickened.

Reduce heat to low and stir in heavy cream, warming through without boiling for 3-4 minutes.

Season with salt, black pepper, and optional nutmeg to taste, adjusting as needed for your preference.

Stir in fresh chopped parsley and optional thyme leaves just before serving for maximum freshness.

Ladle hot soup into warmed bowls and garnish with additional parsley, mushroom slices, or a drizzle of cream.