Preheat the oven to 220C/425F/Gas 7.
For the salad heat the oil on a medium heat in a small frying pan. Add the walnuts and fry until golden brown. Place the walnuts into a bowl with the other salad ingredients, mix together and place onto a serving plate.
Meanwhile, for the croutons, melt the butter in a clean ovenproof frying pan and add the ciabatta pieces, stirring well to coat in the butter. Sprinkle over the cheese and transfer to the oven to bake for three minutes, or until the cheese has melted.
For the dressing, whisk together the ingredients in a bowl. Drizzle over the salad and gently turn over to coat the leaves.
To serve, scatter the croutons over the salad.