Peel and finely chop the onion and parsnips. Warm a pan for 1 min over a low heat. Add 1 tbsp oil and the chopped veg. Add in most of the thyme sprigs (keep 1 back for garnishing). Season with salt and pepper. Cover and gently cook for 10 mins to soften the veg.
Meanwhile, peel and quarter the pear. Slice out the core and dice it. Crumble the stock cube into a measuring jug. Add 800 ml boiling water and stir to dissolve (or warm up 800 ml homemade stock).
Stir the pear into the veg. Pour in the hot stock. Cover and bring to the boil. Simmer for 8-10 mins till the veg are soft when pressed with a fork. Fish out the thyme sprigs.
Add the spinach to the pan. Simmer for 2 mins without the lid on the pan (this keeps the spinach green). Ladle the soup into a blender and blitz it till smooth, or use a hand-held blender in the pan. Taste and add more salt and pepper if it needs it. Ladle into warm bowls and garnish with fresh thyme leaves.