Trim the base and dry tops from the fennel bulbs. Pick the fronds off the bulb and set aside. Halve the fennel and thinly slice it. Peel and finely chop the garlic clove.
Warm a large frying pan to a medium-high heat. Add the cashews to the hot pan and fry for 2-3 mins, tossing frequently, till the nuts are golden brown and toasted. Tip into a bowl and allow to cool slightly.
Pop the cashews into a small food processor or blender. Slide in the chopped garlic. Add the mint and parsley leaves. Grate in the lemon zest and squeeze in the juice. Pour in 2 tbsp oil and sprinkle in a pinch of salt and pepper. Blend into a pesto. No food processor or blender? Finely chop all the ingredients then grind them together in a pestle and mortar, or chop everything together as finely as you can. The pesto will still be delicious if it’s a little chunky.
Return the empty frying pan to a medium-high heat. Pour in 1 tbsp oil. Slide in the sliced fennel. Add a pinch of salt and pepper and cook for 10-12 mins, turning occasionally, till caramelised.
Drain and rinse the butter beans. Shake off any excess water and add to the fennel. Stir well, and cook for a further 5 mins, till the fennel is tender.
Tip the fennel and beans into a large bowl. Toss in the lamb’s lettuce. Spoon over the cashew pesto and fold to combine. Divide betwen two plates and serve straight away.