Prepare the ingredients: chop onion and celery, clean and slice mushrooms, dice garlic, pick thyme leaves.
Heat olive oil in a large saucepan, add onion, celery, garlic, cook for 2-3 minutes over a medium heat, until softened.
Add 600g sliced mushrooms and thyme leaves and cook for 5-10 minutes, stirring occasionally, until all the water evaporates and the vegetables are slightly golden-brown.
Pour stock into the pan and bring to the boil over a high heat. Simmer for 15 minutes turning the heat down to low.
Melt butter in a skillet, add the remaining mushrooms, salt and freshly ground black pepper and cook for 10-15 minutes until golden-brown. Set aside.
After simmering, season to taste with salt and freshly ground black pepper, then whiz with an immersion blender until smooth.
Add heavy cream, bring back to the boil, then turn off the heat.
When serving, garnish the soup with fresh thyme leaves and some of the mushrooms made separately.