This is a really easy recipe, it needs to cook down for an hour or so but can pretty much be left to its own devices for that time. 

Ingredients
500g of leeks or mushrooms depending if you want a lighter or darker gravy 
250g frozen soffritto (or 1 onion, 1 carrot and 1 celery stick)
200ml wine (Prosecco or white for leeks, red or Marsala for mushrooms) 
1 litre vegetable stock 

Method
Brown the leeks or mushrooms on a medium heat (this will take around 10 minutes). Add the chopped vegetables and cook for a further 5/10 minutes until soft. Add the alcohol and bring to the boil to burn off the alcohol. Add the stock, bring to the boil then lower the heat and cook for 1/1½ hours stirring occasionally until thick, reduced and full of flavour.  

Strain, pushing against the vegetables to extract every bit of flavour you can.  Heat, season and serve.