Peel and finely chop the onions. Peel and crush or finely grate the garlic.
Pour 1 tbsp oil into a pan and bring to a medium heat. When hot, slide in the onions and garlic. Stir in 1 tsp turmeric and a good pinch of salt and pepper. Fry for 5 mins, till softened and golden.
Tip the lentils into the pan and crumble in the stock cube. Top up with 750ml boiling water and stir well. Pop a lid on the pan and return to the boil, then reduce to a simmer. Cook for 20 mins.
Meanwhile, roughly shred the baby leaf spinach. When the lentils have 5 mins remaining, stir in the spinach. Pop the lid back on the pan and cook for 5 mins, till the leaves are wilted.
Grate the lemon zest and set aside. When the spinach has wilted and the lentils are tender, squeeze in the lemon juice. Use a stick blender to whizz the soup till smooth. Alternatively, pour into a blender (in batches if necessary), and blend well.
Pour the soup into bowls serve with a sprinkle of the lemon zest.