Roast Chicken, Walnut and Grape Salad with Cheese Dressing

Ingredients

• 1 chicken (about 1.6kg)
• 20 gm softened butter
• 1 tbsp olive oil
• 150 gm seedless red grapes, halved
• ½ celery heart, thinly sliced widthways, leaves reserved
• ½ red apple, unpeeled, thinly sliced
• 100 gm walnut halves, toasted
• To serve: flat-leaf parsley leaves
• Blue cheese dressing
• 3 egg yolks
• 1½ tbsp tarragon vinegar
• 1 small garlic clove, finely chopped
• 220 ml light olive oil
• 80 gm blue cheese, such as Stilton
• 1 tbsp lemon juice, or to taste