I thought châteaubriand had a veal jus and maybe mushrooms?

 Châteaubriand can have a tiara and diamond earrings, if you want. Recipes change quite a bit in the course of 200 years.

As mentioned elsewhere at Ochef châteaubriand was traditionally served with potatoes pared down to the size of large olives (parboiled and finished in butter) and sauce châteaubriand.

The sauce châteaubriand is made with thickened veal stock and "mushroom peelings," so you are quite possibly on the right track. It also includes shallots, thyme, bay leaf, white wine, butter, lemon juice, parsley, salt, pepper, and tarragon. It is rarely made anymore.

As mentioned in the same Ochef article, when you order châteaubriand in a restaurant these days you are quite likely to get a porterhouse steak. So the likelihood that you received some cut of beef in a restaurant served with veal jus and mushrooms would not surprise us at all. If you got a real châteaubriand with real sauce châteaubriand, then you were treated very well, indeed.

In the last century, even restaurants that serve real châteaubriand generally serve béarnaise sauce alongside (which is not to be sneezed at, but it ignores at least 100 years of tradition).

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