Pour 600ml of water into a saucepan and bring to the boil, add 10g of dried porcini then remove from the heat, cover with a lid and leave to soak for 30 minutes. At the end of that time you will have a deeply flavoured mushroom stock.
Set the oven at 200C. Wash 500g of sweet potatoes then slice thickly and place on a nonstick baking sheet. Pull the leaves from 4 sprigs of thyme. Chop the leaves roughly then stir them into 4 tbsp of olive oil, together with a little salt. Brush the seasoned oil over the surface of the potato slices, grind over a little pepper and bake for about 30 minutes until tender and soft enough to easily crush. They should be lightly crisp on top.
Peel and roughly chop an onion. Warm 2 tbsp of olive oil in a saucepan, then add the onion and let it cook, with the occasional stir, for 15 minutes, or until completely soft and sweet. Stir in 1 heaped tbsp of plain flour, letting it cook for 3 or 4 minutes, then pour in the stock, holding the mushrooms back. Roughly chop the mushrooms and stir them in.
Leave the onion and stock mixture to bubble gently over a moderate heat, stirring from time to time, for about 5 minutes or until it has reduced to a thick gravy. Place the sweet potato slices on plates, spoon over the porcini gravy then add a large spoonful of sour cream or yoghurt to each. Enough for 2.
It is essential to cook your onions until they are deep golden brown and soft to the touch. This will give them the necessary sweetness to act as the base of the porcini gravy. When your gravy is reducing, watch it carefully until it is thick enough. Five to 10 minutes should do it.
This gravy makes a splendidly wintery sauce for sausages and chops. Either grill them or cook in a little oil in a frying pan, then add them to the bubbling gravy. Serve with sweet potato mash.