Put the stock in a large saucepan, add the polenta and stir vigorously. Cook for 5 minutes until thickened and season to taste. Line a 23 cm (9 inch) springform cake tin with a circle of baking paper and grease the sides. Pour in the polenta. Place in the freezer for 10 minutes until cold and solid.
Remove the polenta from the tin and cut into wedges. Brush over the wedges with 2 tablespoons olive oil, place under a preheated hot grill and cook for 2–3 minutes on each side until golden.
Transfer to serving plates and scatter with the rocket leaves and walnuts. Crumble over the goats' cheese, drizzle over the remaining olive oil and spoon some marmalade on to each plate.