Heat the oven to 200°C/Fan 180°C/Gas 6. Use a sharp knife to slice the chicken breasts into 1 cm thick strips. Pop in a plastic freezer or sandwich bag. Add 1 tbsp plain flour and a pinch of salt and pepper. Shake well till the chicken is coated.
Beat the egg in a shallow dish. Tip the breadcrumbs into another shallow dish. Dip each chicken strip first in the egg, then roll in the breadcrumbs to coat them. Pop on a large plate. Cover with cling film and chill in the fridge.
Peel the parsnips and sweet potato. Slice both into chunky chips. Chuck into a roasting tin. Drizzle over ½ tbsp oil. Season. Toss the chips together. Roast for 20 mins.
Grease another roasting tin (or baking tray). Spread the breaded chicken strips out on the tray. Roast for 10 mins on the shelf below the sweet potato and parsnip chips.
Tear a quarter of the cabbage into bite-size pieces (see our tip for what to do with the rest). Slice out any thick stalks.
Stir the cabbage into the roast vegetables. Return to the oven on the bottom shelf and put the chicken on the top shelf. Bake for another 10 mins till the veg and chicken are golden and cooked through.
Spoon the Thai sweet chilli sauce into a serving dish. Serve with the chicken strips and rainbow chips for dipping.