Traditional Dutch meat jus (Hollandse Vleesjus) prepared from deep golden pan drippings of roasted beef or meatballs, deglazed with rich beef stock and cold butter, spooned into the crater of hearty potato stamppots.
After removing browned meatballs or roasted meat from the pan, place the pan over medium heat with the reserved pan drippings, scraping the browned bits (aanbaksels) free with a wooden spoon.
Pour the hot beef stock into the sizzling pan, whisking vigorously to incorporate all the concentrated savoury fond from the bottom into a dark, flavourful broth. Bring to a brisk simmer.
Whisk in the teaspoon of mustard for depth and acidity. Simmer gently for 6 to 8 minutes until slightly reduced and flavourful. If a glossier coat is desired, whisk in the cornflour slurry and simmer for 1 minute.
Take the pan off the heat. Whisk in the cubes of cold butter one by one until the jus takes on a rich sheen and velvety mouthfeel. Season with salt and pepper, pouring immediately into a warm gravy boat.