Add sliced mushrooms, chopped onion, and minced garlic to the crockpot.
Pour in chicken or vegetable broth and add olive oil or butter.
Season with salt, black pepper, and dried thyme. Stir gently to combine.
Cover and cook on LOW for 6–7 hours or HIGH for 3–4 hours, until mushrooms are tender and flavorful.
Stir in heavy cream or half-and-half during the last 30 minutes of cooking.
Optional: Blend part or all of the soup using an immersion blender for a creamier texture.
Taste and adjust seasoning before serving. Garnish if desired.