A home-made ranch dressing made of mayonaise, Minced basil, minced oregano, a little garlic powder, minced onion, maybe a hint of dill weed, and salt and pepper make a great sauce for stuffed mushrooms.
Spinach dip would work very well with this also, cream cheese, chopped water chestnuts, minced onion, and chopped fresh spinach.
As stated before, a good marinara, basil based, would be good, as would a pesto.
Get creative and make a cheese fondue to dip the mushrooms in. That would be tasty.
Here's a unique idea, a good beef au jus, thickend with a little cornstarch slurry, and ladled over the mushrooms.
I'm not familiar with Kosher rules, and so please excuse my ingnorance if I have included anything that is not Kosher.
Another good option would be a Mornay Sauce.
In answer to your question, Hunter's Sauce -
http://www.food.com/recipe/hunters-sauce-with-mushrooms-84564
Seeeeeeya; Chief Longwind of the North