Put the potatoes into a large saucepan. Cover with boiling water and add a pinch of salt. Cover and simmer for 15 to 20 minutes, until tender.
Meanwhile, put the frozen mince into a large saucepan and add the carrot, onion, parsnip or turnip and courgette. Measure in 450ml (3/4 pint) boiling water, then simmer for 15 minutes without a lid.
Preheat the oven to 200 C / Fan 180 C / Gas 6. Drain the potatoes, then mash them with the milk and season with black pepper.
Sprinkle the gravy granules into the mince mixture, stirring until thickened. Transfer to a baking dish and top with the mashed potato. Add the butter in a few tiny knobs (if using), then bake for 20 to 25 minutes until golden brown.
Just before the pie is ready, make up some more gravy by sprinkling 4 heaped teaspoons of Bisto® Vegetable Gravy Granules into a jug, adding 280ml (1/2 pint) boiling water, stirring until thickened and smooth.