One of my favorite things to do is go buy the cheap beef roasts at the local grocery store (I trust the folks who work in the meat dept) and put them in the water bath at 138F for about 48 hours (they are good at 25, excellent at 48). I did an eye of round and a bottom round this last week… and I’ve got a top round going right now. Look for sales and freeze extra roasts for later.
If it is a very lean cut (e.g. bottom round) then give some thought to adding fat from one of the more fattier cuts (1/2 cup duck fat would make you a hero in my house). The protein will pick up the taste of the fat you cook it in.
If you use butter then go with the cheap stuff (grass-fed is going to give it a sharp acidic note) or just throw in some olive oil, rosemary, and salt and make it into a proper feast. 
Good luck.
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