Heat your oven to 180°C/Fan 160°C/Gas 4. Scrub the potatoes and slice them into chips about as big as your little finger. Pop them in a bowl with ½ tbsp oil and a pinch of salt and pepper. Toss them together to coat. Set aside.
Make your crab stuffing. Trim the roots and top 1cm off the spring onion. Finely slice it. Peel and grate or crush the garlic. Halve the chilli and flick out any seeds or white pith for less heat, then finely chop the chilli. Finely grate the zest from the lemon. Pick the leaves off the thyme sprigs (if the stems are woody, you can run a fork down them to pull the leaves off). Chop the thyme leaves a few times to roughly chop them.
Line a baking tray with baking paper and spread the chips out over it. Bake the chips on the top shelf of the oven for 15 mins.
While the chips bake, warm a medium pan over a medium heat for 1 min, then add the spring onion with 2 tbsp water. Season with a pinch of salt and pepper. Fry and stir for 5 mins till the spring onion starts to soften.
Stir the garlic, chilli, lemon zest and most of the thyme leaves into the spring onion (keep a pinch back for garnishing). Cook and stir for 1 min till the pan is fragrant, then stir in the crab meat, breadcrumbs and 1 tbsp Dijon mustard. Stir well to mix everything together.
Take the crab off the heat, taste and add a pinch more salt and pepper if you think it needs it. Spoon the crab stuffing into the shells and pop them on a baking tray.
The chips should have cooked for 15 mins by now. Move them down to the middle shelf and pop the crab on the top shelf. Bake for 15-20 mins, till the they're browned and the chips are tender and golden.
While the crab and chips cook, toss the lamb's lettuce with the juice of half the lemon and some salt and pepper.
Take the crab out of the oven and squeeze over the remaining lemon halves. Serve the stuffed crab with the chips and lamb's lettuce on the side.