Trim the very ends off the leek and halve it. Rinse out any grit and finely chop it. Warm a large pan over a medium heat for 1 min. Add 1 tbsp olive oil and the leek. Season with salt and pepper. Cover. Gently cook over a medium-low heat for 5 mins till the leek is soft. Stir now and then.
While the leek sweats, peel and crush the garlic. Break the broccoli into small florets. Roughly chop the stem, discarding the dry end. Stir the garlic into the leek with a pinch of chilli flakes (use as little or as much as you like, they're spicy). Add the broccoli.
Crumble in the stock cube. Pour in 800ml boiling water (or just use 800ml homemade stock). Turn the heat up and bring to the boil. Simmer for 10-12 mins, without the lid on, till the broccoli is soft. Ladle into a blender and blitz, or use a handheld blender to blitz it.
Zest the lemon. Squeeze the juice into the soup and taste it. Add more salt and pepper if you think it needs it. Ladle into a bowl and garnish with the lemon zest to serve.