The best funghi porcini I have ever bought in London (oh, sod it – they're better than any I've ever picked myself too) come from François' stand at the Duke of York Square market; he's there on Saturdays with a whole host of other French produce from onions to cheese. For other markets he goes to see: @LaFermeLondon.
This week there are no cepes as it was too rainy in France so those available have bugs, but he is selling beautiful chanterelles.
Whether or not to wash funghi porcini is a matter of preference – most people will tell you to just brush the dirt off, but I think there are times when a quick rinse and wipe is fine. I also trim the tougher bits.
Put a large pan of well salted water on to boil.
I slice the mushrooms. Then I warm olive oil in a pan (be generous as this will be the 'sauce'), and first sautee the stems. Then I throw in a generous pinch or four of sea salt. Most people used fresh garlic, but I find it tends to singe in this recipe, so instead I used half a teaspoon of powdered garlic or garlic salt (reduce the sea salt accordingly). Then I add the sliced tops of the mushrooms and thrown fresh tagliatelle into the now boiling water.
The mushrooms are ready when they have all soaked up some of the oil, roughly when the pasta is ready. Throw on some chopped flat parsley, mix and serve.
Incidentally, the mushrooms will not freeze when raw, but do freeze well when cooked this way.