Tip the pearl barley into a pan and stir in 300ml boiling water and a pinch of salt. Return to the boil, then pop a lid on the pan and reduce to a simmer. Cook gently for 20 mins, till the grains are soft with a little bite.
Meanwhile, peel and thinly slice the red onion. Slide into a bowl and pour over 1 tbsp red wine vinegar. Sprinkle in a little salt and scrunch everything together till well coated. Set aside to lightly pickle.
Fill a pan with water and bring to the boil. Cut the cauliflower into small florets. When the water is boiling, slide in the cauliflower and cook for 4-5 mins till just tender. Drain well and allow to steam dry. Mix 1 tbsp each Cajun seasoning and cornflour in a large bowl and stir in a pinch of salt and pepper. Add the warm cauliflower florets and toss till coated with the spice mix.
Pick the mint leaves and finely chop them. Finely chop the parsley, leaves and stalks. Peel and crush or finely chop the garlic. When the barley has cooked, drain if necessary then scatter into a bowl. Fold through the chopped herbs, crushed garlic and 1 tbsp oil.
Pour 2 tbsp oil into a pan and set on a high heat to warm up for 2 mins. Carefully add the coated cauliflower florets and fry for 4-5 mins, turning occasionally till crispy and golden on all sides.
Spoon the herbed barley onto plates and top with the crispy Cajun cauliflower. Finish with a scattering of rocket leaves, a crumble of feta and the pink pickled onions and vinegar.