Cook the potatoes and swede in simmering water until tender, about 20 minutes
Meanwhile, heat a large saucepan and add the mince, a handful at a time, cooking it until browned. Stir in the onion, carrot, green beans, mushrooms and mixed herbs. Add 450ml water and simmer without a lid for 20 minutes, adding the peas for the final 2-3 minutes.
Preheat the grill and warm a large baking dish under it for 1-2 minutes. Sprinkle the gravy granules into the mince mixture, stir until thickened, then transfer to the baking dish.
Drain the potatoes and swede, season, then mash thoroughly. Stir in the chives and about three-quarters of the cheese. Spoon over the mince, spreading it out to cover it. Sprinkle the remaining cheese on top, then grill until golden brown. Serve.