Halve, peel and finely dice your onion. Finely dice your celery. Set a pan over a high heat. Add 1 tbsp oil, the diced onion and celery, plus 1 tbsp cumin. Season well. Lower the heat and pour in 750ml boiling water. Simmer for 5 mins.
Take the veg off the heat. Set aside to steep and cool for 10 mins. Peel, core and roughly chop your apples. Roughly chop your spinach, removing any large stems. Add the chopped apple to a blender or food processor with the spinach.
Start blending, pouring in the slightly cooled liquid till it's as thin or as thick as you like. Taste. Whizz in the stock cube or more salt, pepper or cumin if you think it needs it. Finish with a little lemon zest and juice for extra zing.
TIP: The cooler the liquid is, the greener your soup will be. Then you can gently reheat it once everything is blended.