I ended up freezing the leftover jus to create this dish again. This is a good thing since the restaurant where we had the original dish changes their menu weekly so I can't count on having this dish again.
1 cup lamb jus - leftover from roasting a leg
1 lb linguini
2-3 cups mushrooms - I used a combination of cremini and porcini, sliced
1 T olive oil
1 t garlic
1/3 cup heavy cream
2 T butter
1/2 t salt
1/2 t pepper
1 cup leftover cooking water from the pasta
While linguini is cooking, saute mushrooms in olive oil until softened. Add garlic, salt, and pepper. Saute another minute. Add lamb jus and butter. Once the butter has melted, turn down the heat and add the cream. Season to taste with additional salt and pepper if necessary.
Drain pasta, add half of the reserved cooking water. Add mushroom sauce and additional cooking water if the sauce looks tight and serve immediately. Garnish with fresh Parmesan cheese.