Trim the roots and top 3cm off the leek. Halve it, rinse out any grit and finely slice it. Warm a large pan over a medium-low heat for 2 mins, then add the leek with ½ tbsp oil and a pinch of salt and pepper. Fry for 4-5 mins, stirring occasionally, till the leek has started to soften.
While the leek fries, trim the dry ends off the celery and finely slice it. Finely chop the courgette, or coarsely grate it. Stir the celery and courgette into the leek. Fry for another 5 mins, stirring now and then, till all the veg have softened a little.
While the veg fry, peel and crush or grate the garlic. Finely grate the zest from the lemon. Crumble the stock cube into a heatproof jug and pour in 450ml boiling water. Stir to dissolve the stock cube.
Add the garlic and lemon zest to the pan along with the pearl barley. Give everything a good stir to mix it, then pour in the stock. Pop a lid on the pan, turn up the heat and bring to the boil. Once it's boiling, turn the heat back down and simmer for 15 mins till the barley is almost cooked – it should still be a little chewy when you bite into a piece.
While the barley simmers, pick the soft leaves off the tarragon sprigs and roughly chop them. Snap the woody ends off the asparagus and chop the tender tips into 1cm chunks.
Add the asparagus and most of the tarragon leaves to the pan. Give it a good stir, then simmer, without the lid, for 3-4 more mins. The pearl barley should be tender, but still with a little chew, and most (but not all) of the stock absorbed.
Squeeze some lemon juice into the risotto. Taste it and add more salt, pepper or lemon juice if you think it needs it. Spoon the risotto and its stock into warm bowls. Top with the lamb's lettuce to serve.