Peel and finely slice the garlic clove. Warm a pan over a low heat. When warm, pour in 1 tbsp olive oil. Add in the sliced garlic with a pinch of salt. Fry for 3-4 mins, till the garlic slices are golden and crispy. Remove from the pan with a slotted spoon, popping them onto some kitchen paper to drain.
Peel and finely chop the onion. Peel and finely dice the potato. Place the pan back on the heat. Add the onion and fry for 5 mins in the garlicky oil. Add the potato and cook for 3 mins. Crumble in the stock cube. Pour in 700ml boiling water. Cover. Simmer for 10 mins.
Drain and rinse the cannellini beans. Add them to the simmering pan. Roughly chop half of the baby leaf spinach, and set the rest aside. Add all the chopped spinach to the pan. Grate in the zest from the lemon, then halve it and squeeze in the juice from 1 half.
Blitz the soup till smooth using a stick blender, or transfer to a blender to blitz. Put the soup back on a low heat. Swirl through the reserved spinach. Taste and season the soup, adding more lemon juice if needed. Serve in warm bowls topped with the crispy fried garlic.