Preheat your oven to 200°C/Fan 180°C/Gas 6. Peel and discard the papery outer layers of the whole garlic bulb, leaving intact the skins of the individual cloves of garlic. Place the garlic bulb in a large roasting tin and slide it into the oven while it warms up.
Meanwhile, trim, peel and chop the parsnips. Remove the tin from the oven and add the parsnips. Add ½ tbsp olive oil and some salt and pepper. Slide into the oven for 15 mins.
While the parsnips roast, rinse the mushrooms well, shake dry and tip them into a bowl. Pour in ½ tbsp olive oil and add some salt and pepper. Turn the mushrooms to coat them.
When the parsnips have cooked for 15 mins, add the mushrooms to the tin. Slide back into the oven for 15 mins. Fill and boil the kettle.
Remove the tin from the oven, carefully lift out the garlic and set it to one side to cool. Place the roasting tin on a medium heat and carefully pour in 700ml hot water. Add most of the thyme leaves and simmer for 10 mins. If you prefer, you can tip the roast veg into a medium pan then pour in the hot water and add the thyme.
While the soup simmers, carefully pull apart the roast garlic cloves and squeeze the flesh from half the bulb out of their papery skins. Add the peeled garlic to the simmering veg. When the soup has simmered for 10 mins, use a stick blender to whizz the soup smooth, or ladle the soup into a blender and blitz it smooth. Spoon in half of the crème fraîche and blend again. Taste the soup and add more roasted garlic if you like, or more salt or pepper if you think it needs it.
Ladle the soup into a couple of warm bowls. Top with the remaining crème fraîche and scatter over the remaining thyme leaves to serve.