Easy one for if you have some left overs in the bottom of the fridge.
You will need:
1 small tub sour cream (or creme fraiche)
1/2 tub of philadelphia (other cream cheeses are available from all good stores)
4-6 mushrooms (doesn't matter if they are starting to look like testicles)
As much pasta as you like, of whatever type you like
2 dessert spoons of parmesan cheese
pepper to taste.
Cook the pasta as per instructions on the packaging.
Slice the mushrooms and fry them off in a little butter, oil, or whatever you like.
Drain the pasta but allow enough water to stay in the pan just to cover the bottom of the pan to put in the sauce so that it sticks nicely to the pasta.
Into the small amount of pasta water, add the phili, sour cream, mushrooms, half the parmesan and some pepper to taste.
Once it starts to melts down, add the pasta and stir so that it is covered evenly.
Place into a serving dish and sprinkle the remaining parmesan over the top.
Average cost if you have to buy stuff in, £4. If you have it all lurking in your fridge, nada and you've cleared the fridge