Sauté the vegetables: In a large pot, melt the butter over medium heat. Add the diced onion and cook until softened, about 5 minutes.

Add mushrooms and seasonings: Stir in the sliced mushrooms and thyme. Cook until the mushrooms are golden brown, around 7-10 minutes, stirring occasionally.

Add broth and simmer: Pour in the chicken broth and bring the mixture to a boil. Reduce the heat and let the soup simmer for 15 minutes, allowing the flavors to meld.

Blend the soup: Use an immersion blender to puree the soup until smooth and creamy, or leave some chunks for added texture, depending on your preference.

Finish with cream: Stir in the heavy cream and season the soup with salt and pepper to taste. Allow it to heat through but not boil.

Serve and garnish: Ladle the soup into bowls, garnish with freshly chopped parsley, and enjoy hot.