A vegetarian dish which I use when I'm tired of meat. Simple to make and tasty. From Leith's Recipe of the Day. My children like it, my husband loves it. Can be made ahead and reheated. It will take 40 minutes to reheat from cold.
Bring a large sauce of salted water to the boil, add the butternut and sweet potatoes and cook for ten minutes or until tender. Drain well.
Meanwhile, melt the butter in a saucepan, then add the flour, mustard and cayenne pepper. Stir to combine and cook for one minute. Take the pan off the heat and add the cold milk gradually, stirring all the time.
Return the saucepan to the heat and bring to the boil, stirring, until the sauce is thick. Allow to simmer for two minutes. Remove from the heat, then add parsley and all but 2 tablespoon of the gruyere cheese. Season to taste.
Add the squash and sweet potatoes to the white sauce and stir to coat thoroughly. Tip the mixture into a baking dish.
Make the topping by mixing together the breadcrumbs, Parmesan and remaining Gruyere cheese. Sprinkle over the sweet potato and squash filling.
Bake in the oven for 20 - 30 minutes until bubbling and golden on top.