Heat the oven to 200ºC/Fan 180ºC/Gas 6. Set the cashews on a small baking tray. Bake for 4–5 mins till they turn a pale golden brown. Keep a close eye on them, as they can burn easily. Set aside to cool. Pick the leaves off the coriander stalks.
Scrub the potatoes and halve or quarter them, depending on their size. Peel the carrots. Cut them on the diagonal in chunky slices about 3 cm thick.
Pop the carrots and potatoes in a roomy roasting tin with the chicken thighs and garlic cloves still in their skins. Season. Drizzle with 1 tbsp olive oil. Roast for 30 mins.
Slice the pepper into lengths. Cut away the white bits and seeds. Trim the spring onions and cut into chunky pieces. After 30 mins, add the pepper and spring onions to the roasting tin. Roast for 30 mins till everything is golden and cooked through.
Remove the tough outer layer of the lemon grass. Chop the rest of the lemon grass roughly. Empty it into the small bowl of your food processor. Add the cashews and whizz to chop finely.
Add the coriander leaves and 3 tbsp cold water to the processor. Whizz again to make a thick pesto. Scrape the pesto into a small serving bowl and stir in 2 tbsp olive oil. Season to taste.
When the chicken and vegetables are cooked through, divide them between 2 plates and serve with a couple of dollops of the coriander and lemon grass pesto.