How to fix grass finished beef that tastes like fish?
Grass-fed (and finished) beef fat tastes fishy because it effectively is similar to fish oils.  In particular, grass-fed beef fat is dramatically higher in omega-3 fatty acids (alpha-linolenic acids) as opposed to omega-6 (linoleic) fatty acids; this NIH funded study for example found:


  ... overall average of 1.53 and 7.65 [omega-6 to omega-3 ratio] for grass-fed and grain-fed, respectively, for all studies reported in this review.


Meaning that grass-fed beef might have a 3:2 ratio of o6:o3 fatty acids, while grain-fed beef has a 13:2 ratio - a ratio over 4 times higher.

Fish tends to have even more omega 3 fatty acids, tending to a ratio of less than 1 (so, more omega 3 than omega 6); so I wouldn't imagine your beef tastes entirely like fish.  But you're definitely going to taste some 'fishiness' in there in grass-fed and grass-finished beef as compared to grain-finished or grain-fed.



As far as how to prepare it, the challenge with beef is the strong beef flavor that you don't want to entirely cover up (or you wouldn't be buying a nice grass-fed side of beef, you'd be buying something from the local megamart).

My suggestion is to take your cue from fish itself: specifically, from strong tasting fish, and fish that is commonly eaten in steak-like preparations.


Steak Tartare or Beef Carpaccio is one suggestion.  By not melting the fat, you're not going to release as much of the 'fishy' flavor, and in fact this is a common preparation for fishier fish as well (Tuna and Salmon in particular).
Add an acidic sauce, such as hollandaise.  Acidic flavors cut the fishy flavor well, just as spritzing some lemon juice onto your salmon does.
Braised preparations such as how short ribs are commonly prepared work well because not only can you introduce an acidic flavor but you also allow some of the oil to be extracted from the fat and blended into the meat where it's not quite as concentrated as simply eating the steak.  While braised beef can be very heavy on the sauce, a simple braise in red wine might be effective at counteracting the oil flavor while still keeping the beefiness.
Removing the fat before cooking (trimming) will work well for cuts that have solid fat. Even something like a ribeye (which has a lot of both marbling and solid fat), simply doing this might be sufficient if the taste doesn't bother you too much.
Frying in butter, or in a fat that is not high in omega-3 fatty acids, may help as well.  Olive oil should be fine (very little omega-3); avoid flaxseed oil or blended oils that list omega 3 fatty acids as a health benefit. This probably won't help with the solid fat, though, only with the marbling.
Finally, you will get used to this taste just as you did with gaminess.  So, simply eating it and not trying to alter it may be the right path if you are going to continue buying grass-fed and grass-finished beef in this manner.  Many tastes are possible to get used to after five or six meals of them.