Truffles and potatoes:
Truffles were born to be the star. Even in truffle oil, it's better to use them in a way where they can stand out rather than force them to be a team player. Truffle oil, almost always works best with something simple. Make a warm potato salad with plenty of herbs and the truffle oil.
A similar idea, but one that gets the health food in, is to make a composed salad of some green vegetable -- such as runner beans -- and the potatoes. Otherwise, you're on your own when it comes to inserting the roughage.
Serve the potato bean salad immediately before the fish. You could do the same with the straight potato salad, or serve it on a side plate along with some sort of separately prepared, wilted greens. A soup plate and side plate is a little weird, but that's what I'd do.
Alternatively, prepare either latke or "Swedish" style potato pancakes with the russets (which make excellent pancakes). Latkes use grated potatoes, and Swedish style use ground. Latkes use less flour than "Swedish" style. And when I say Swedish I don't really mean Swedish -- just generic western, northern European. Mound a tartare of smoked salmon dressed with truffle oil and capers. Serve as appetizers. Note: Potato pancakes are no good unless they're straight out of the pan. To serve sixty, even staggering service a little, you'll have to keep four 12" pans going at once. You can't do it without help. At least I can't.
Serve with "frozen" aquavit or vodka.
For the fish:
Roast the cod. Separately prepare a rough and ready sort of boullibaise; don't knock yourself out, it's only a sauce. Include in it some fennel and/or Pernod. Separately steam some shellfish -- edit heavily, not too many species -- only whatever is the most attractive and best available. Prawns and clams would be very good.
Strain the broth into soup plates, and plate the cod on top of the broth. Make sure there's enough broth to sop up later; but not so much that the dish appears to be a soup. Garnish with the steamed shellfish. Nappe the cod with the aioli, and/or pass the aioli so the guests may serve (or add to) themselves.
Whether or not you serve the potato salad with the cod, bread is very important with your "deconstructed soup." Either simple, sour and/or pain de campagne baguettes. A dozen baguettes should do it. Serve them whole, by laying them on the table where the guests may reach and break them. Let the crumbs and bits of crust fall where they may.