Slice the mushrooms, dice the onion, and mince the garlic.
For extra flavor, heat the butter or olive oil in a skillet over medium heat.
Sauté the onion for 3-4 minutes until softened.
Add the garlic and cook for 1 minute until fragrant.
Transfer the sautéed mixture to the crockpot.
Add the mushrooms, broth, thyme, salt, and pepper to the crockpot.
Stir to combine.
Cover and cook on LOW for 6-7 hours or HIGH for 3-4 hours until the mushrooms are tender.
For a smoother texture, use an immersion blender to puree part or all of the soup. (Alternatively, transfer a portion to a blender and return to the crockpot.)
Stir in the heavy cream and let the soup heat through for another 10-15 minutes on LOW.
If you prefer a thicker soup, add the cornstarch slurry and stir until the soup thickens.
Ladle the soup into bowls, garnish with chopped parsley or croutons, and serve warm with bread or crackers.