Cook the potatoes and butternut squash in a large saucepan of simmering, lightly salted water for about 20 minutes, until tender.
Meanwhile, heat a large saucepan and spray with calorie-controlled cooking spray. Add the onion, carrot, celery and swede or parsnip, stir-frying them for 4-5 minutes. Add the Quorn mince and vegetable stock. Cook over a low heat for 25 minutes, adding the peas or sweetcorn at the end.
Preheat the grill. Warm a baking dish with a capacity of about 1.5 litres (2 ¾ pints) under the grill.
Drain and mash the potatoes and butternut squash, then season with a little salt and pepper. Blend the cornflour with 2 tablespoons of cold water and add to the Quorn mixture, stirring until thickened. Tip the mixture into the baking dish and spoon the potato mixture on top to cover the surface. Grill for about 5-6 minutes until browned. Serve at once.
Tip: Prepare ahead and keep chilled, then bake in the oven preheated to Gas Mark 5/190°C/fan oven 170°C for 25-30 minutes.