Preheat oven to 180°C/160°C fan forced. Grease a 20cm flan tin.
Place cheese sauce mix, flour and butter in a food processor and process until combined. Add sufficient water to make a dough. Wrap in plastic wrap and refrigerate 15 minutes.
Roll out pasty between 2 sheets of baking paper; ease into prepared tin, pressing into base and sides; trim excess pastry.
Cook spinach in boiling water for 3 minutes; drain and rinse under cool water; squeeze out any liquid.
Place spinach in the pastry case, sprinkle with parsley and cheese. Whisk together eggs and milk; pour over cheese.