This is what i do for a sauce when i'm making steak, you might be able to adapt it using the roasting juices.
Once the steak is out of the pan & resting add chopped onion & garlic to the pan with some water & scrape the bottom of the pan with a spoon or spatula to deglaze
Cook till softened
Add mushrooms if required and cook through.
add seasoning, a splash worcestershire sauce & splash of brandy, cook to evaporate alcohol.
Take off the heat to cool slightly and add a few spoons of quark to the mixture, stirring in to melt.
You can heat very gently after this point if necessary but don't let it get too hot or the quark will split.