1. Preheat your oven to 200°C/Fan 180°C/Gas 7. Peel and finely chop the onion. Halve the pepper and scoop out the seeds. Chop it into small pieces. Place a large pan on a medium heat for 1 min then add the butter, onion and pepper. Season with a small pinch of salt and pepper. Cook for 8 mins, stirring often, till the veg is glossy and starting to soften.
2. While the veg cook, chop the leftover cooked veg into small, bite-sized pieces. Chop and shred the leftover roast turkey (or chicken), keeping it in large pieces. Pop it into a large casserole dish or deep ovenproof dish.
3. When the pepper and onion are glossy, tip them into the dish with the chopped veg and turkey. Pour in the mushroom soup and chicken stock and stir gently together. Taste and add more salt and pepper if you think it needs it.
4. Slide the dish into the oven to bake for 15 mins while you make the biscuit topping.
5. For the biscuits, sift the flour, baking powder and salt into a large bowl. Coarsely grate in the cold butter, then rub in with your fingertips to create breadcrumbs. Add the thyme leaves. Stir in the buttermilk till combined to make a soft dough.
6. Tip the dough out onto a lightly floured worksurface and pat out till the dough is around 11/2cm high. Using a 6cm cutter, cut out 8-10 rounds. Gather the trimmings bakc together and reshape into a 11/3cm dough square as you go.
7. Remove the turkey casserole from the oven and place the biscuit rounds on top. They should cover the turkey pie filling. Brush the top of the biscuits with the beaten egg. Slide into the oven and bake for 20-25 mins till the biscuits are golden and have risen. Serve with greens.
8. When is a biscuits not a biscuit? American biscuits are small, soft baked rounds of dough a lot like British scones. They're almost always eaten with savoury food, like this turkey casserole or with fried chicken, barbecue, country ham, or for breakfast with sausages, bacon and scrambled eggs.