Brush the mushrooms clean, then finely slice. Peel and finely slice the onion, celery and garlic, then pick the parsley, finely chopping the stalks. Pick the thyme leaves. Heat a splash of olive oil in a large saucepan over a medium heat, add the onion, celery, garlic, parsley stalks, thyme leaves and mushrooms, pop the lid on and cook gently until softened. Spoon out 4 tablespoons of mushrooms, and keep for later. Pour the stock into the pan and bring to the boil over a medium heat, turn the heat down to low and simmer for 15 minutes. Season to taste with sea salt and black pepper, then whiz with a stick blender until smooth. Pour in the cream, bring just back to the boil, then turn off the heat. Toast the ciabatta on a hot griddle pan, then top with most of the reserved mushrooms and drizzle lightly with extra virgin olive oil. Spoon the soup into bowls, garnish with the chopped parsley and remaining mushrooms, and serve with the ciabatta crostini on the side.