Peel and finely chop the onion. Trim the dry ends off the fennel and pluck off any feathery fronds for later. Finely chop the fennel. Warm a pan for 1 min, then add 1 tbsp oil and the chopped veg. Season with salt and pepper. Cover, keep the heat low and sweat for 8 mins till the veg have softened a little.
While the veg sweat, peel and grate or crush the garlic. Separate the spring green leaves. Slice out the any thick cores and slice the leaves into thin ribbons. Crumble the stock cube into a heatproof jug. Stir in 1 ltr boiling water (or warm up homemade stock).
Stir the garlic into the veg. Pour in the hot stock. Pop the lid back on, turn up the heat and bring the soup to the boil. Simmer for 10 mins to soften the veg. Add the spring greens to the pan. Simmer without the lid on for 5 mins to just cook the spring greens.
Finely grate the zest from the lemon. Add a squeeze of lemon juice to the pan. Swirl it in and taste. Add more salt, pepper or lemon juice if you think it needs it. Ladle the broth into warm bowls, top with the lemon zest and fennel fronds, and serve.