Place all the gravy ingredients A into a roasting pan or large casserole dish. Drizzle the oil all over. Mix everything up. Roast in the oven for 1 and half hours. The longer roasting time gives a darker and richer gravy
Now, place the roasting dish on the hob on a low heat. Give everything a good stir, scraping the bottom of the pan. If dry add a splash of water and mix it all up.
Add the flour and ginger and work it in to combine well. Leave it "cooking" for about 10-15 minutes to deepen the flavour even more.
Turn the heat up, add the boiling water, lemongrass and lime leaves from B, stir and simmer on low heat for around 20-30 minutes until it's thickened to a gravy consistency.
Check the seasoning then push everything through a sieve in to a clean jug or saucepan, squashing everything through the sieve with a large spoon.
This can be the finished product. However as you've always got a roast going for the gravy, pour the strained juices from whatever roast you have into it and you'll have gravy to die for!
As mentioned, this gravy can be made ahead and kept in the fridge overnight or even frozen. To serve, just add the juices from whatever roast you're having, a splash of port/sherry if you fancy and heat it up thoroughly.