Melt in the mouth walnut and cheese biscuits

Name: Melt-in-the-mouth walnut and cheese biscuits
Ingredients: 75g/2¾oz butter, diced; 30g/1oz semolina; 50g/1¾oz self-raising flour, plus extra for dusting; 50g/1¾oz Parmesan, finely grated; 1 tsp mustard powder; 15g/½oz walnuts, finely chopped; 1½ tbsp beaten free-range egg white; 15g/½oz poppy seeds; salt and freshly ground black pepper