Prepare your veg: peel and finely chop the onion. Trim the ends off the celery and finely slice it. Peel the parsnips and trim off the very ends. Finely chop them.
Warm a pan for 1 min, then add ½ tbsp oil and the veg. Add the bay leaf and most of the thyme (keep a sprig for garnishing). Season with salt and pepper (use white pepper if you have it). Cover the veg with a piece of greaseproof paper. Pop on the lid. Sweat the veg over a very low nheat for 30 mins till they’re soft but not browned – stir once or twice to check.
Lift off the greaseproof paper. Scoop out the thyme sprigs and bay leaf. Pour in 450ml boiling water (or hot homemade stock). Ladle the soup into a blender with the cream and blitz till smooth, or use a hand-held blender in the pan.
Pour the soup back into the pan and warm through for 2-3 mins. Taste and add a little more salt and pepper if you think it needs it. Ladle into warm bowls and garnish with a few thyme leaves to serve.