I'm not 100% sure on the source of this recipe as I came across it in a magazine a couple of years ago, which I no longer have, and I haven't been able to find the source on Google. I do however believe the original recipe was by Matt Moran. I've adapted it slightly. The main change I made was to cook the orzo (risoni) in the soup broth rather than separately to save time and washing up. I suggest slicing the fennel and leek using a mandoline slicer to ensure the thickness (or thinness) is consistent.
Place a large saucepan over a medium heat and add the olive oil and leek. Cook for 4 minutes or until soft. Add the garlic and cook until fragrant before adding the fennel. Cook the fennel for 1 minute before adding the vegetable stock and orzo (risoni) and cook for 12 to 12 minutes, or until the pasta is cooked. Taste and adjust the seasonings to taste.
Ladle the soup into bowls and top with lemon zest, lemon juice and fennel fronds and serve immediately.