Pre-heat oven to 200°C (180°C fan)/400°F (350°F fan)/gas mark 6 (gas mark 4 fan).
Trim off the cauliflower leaves and carefully cut out half the stalk and core, leaving a cavity – the florets should still be holding together. Place the cauliflower into a saucepan and cover with boiling water then simmer for 5 minutes then carefully remove and set aside to dry.
Place the bread and walnuts into a processer and blitz then set aside. Mix the vegetable stock cube with the boiling water in a jug and set aside.
For the stuffing, add half of the olive oil into a saucepan then sweat the onion for 5 minutes. Stir the sliced garlic, sage, kale and add half the breadcrumb mix and half the vegetable stock mix. Cook for 5 minutes. Remove from the heat and add to a processer and blitz until smooth. Allow to cool slightly then spoon into a piping bag/food bag and pipe into the cavity of the cauliflower including in between the florets and place into an oven dish.
Mix the remaining stock with the oil then pour over the cauliflower. Sprinkle over the breadcrumbs and roast for 45 mins until golden brown and tender.