Following the laziest roast crispy duck in history my duck fat jar runneth over … almost. So it should see us through the coming winter unless we take it into our heads to make confit and, considering the mess involved, I rather hope we don’t.
Duck fat is by far the best medium for roasting potatoes or greasing Yorkshire pudding tins and it’s ever my fat of choice for browning winter casseroles or starting off any dishes from the Basque or Languedoc regions. Now I know goose fat is supposed to be even better, but given the price of a goose you might as well resign yourself to buying a tin of it and forego the satisfaction of making your own. But duck fat is different: easier to come by and a most inconvenient waste product to dispose of if you can’t be bothered to save it. If prepared with a modicum of care and kept in a reasonably cool and dark place it will survive for yonks outside the fridge: I keep mine on a shelf in the garage.
Just pour off the rendered fat during and after roasting a duck plain and slow (eg 6 to 7 hours at 140C) and leave it to cool overnight in the fridge. Lift the fat away from any stray juices lurking underneath then reheat it to liquify. Any moisture will bubble away, so when its puttering stops strain the liquid fat through a sieve lined with a couple of layers of muslin or even kitchen roll into a sterilised preserving jar and seal. Discard the brown bits! Enjoy its golden glow fading to white as it cools then hoard and scoop out as needed with a clean, dry spoon.
Depending on size one easily-available Gressingham duck should render at least 250ml fat. If you do nothing else with it, use a couple of tablespoons for roasting potatoes and greasing your Yorkshire pudding tin: you’ll be glad you did.
Gastroplod 
I’ve long been a fan of roasties cooked in goose fat, though when I first started using it, I couldn’t find it in our UK supermarkets (though no doubt some posh delis were already selling it). Instead I’d stock up on a few jars on one of our regular visits to France. Inexpensive and definitely worth it. Nowadays it’s more widely available, certainly my local Waitrose stocks it and I think Tesco do as well. Probably other supermarkets too.
I’ve never roasted a duck nor a goose, my husband not being a huge fan of either, and also having been put off by accounts of goose fat liberally spattering the inside of the oven and taking hellish amounts of cleaning!
Very wise, Kavey, very wise. I think goose roasting is best left to Aga owners as their continual heat burns off spills & splashes. I wouldn’t try it: a friend cooked a goose for Christmas dinner in her gas oven and forgot to pour off the fat; it caught fire and set her chimney alight. Yikes! Luckily Father Christmas had stopped by the night before…