What's the best way to make the broth for chateaubriand?

I prefer béarnaise. How will you be cooking the beef? Stovetop for sear and then oven? agree with lisabu about adding wine and beef stock or even better - demi glace and minced shallots. Start reducing the sauce in the pan you cooked the beef in while scraping the bits from the bottom of the pan with some added fat like butter. Chateaubriand is so lean you may just have fond and a little fat from searing.

Call me old-fashioned, but I still make it with hollandaise sauce. Fluted mushroom caps, double baked potato, a green vegetable, and a big red wine (big taste, big glass!).
Serve on a platter for two and carve at the table.
Save any drippings for another occasion.

Pour off all buts few tablespoons of the fat in the pan then sauté some finely minced shallots til soft. Pour in some red wine and reduced and then add beef broth. Reduce more then add a pat of butter and some parsley.

I'm looking to use the pan juices for a reduction

are you talking about beef stock that is used to make traditional French wine sauces? or do you mean to just use the pan juices in some kind of sauce?