Well, no the browning ship has already sailed. But you can cook it longer and get it tender. Searing it would have locked in some of the juices, but that may not even be why it's tough. Could be the cut of meat, could be the particular animal it's from, could be lots of things. But what I would do is add some onion and spices (if you haven't already), place it in a baking pan like glass with lid or foil, add some water and cook it in the oven. I would cook it at about 300 degrees (F) for a couple hours, minimum. The longer beef cooks (as long as there is some moisture there, so keep adding water if it cooks away!) the more tender it will get. I don't know about eating it right away though.
Other options are to shred it up and use it as pulled beef. It still won't be too tender, but it'll be easier to chew. You can shred it up and add barbeque sauce to it, and make pulled beef sandwiches. Letting it simmer for awhile in the sauce will help a lot too.
Or you could cut it into small cubes, then boil it on the stovetop for a couple of hours, with some beef bouillon and it will get tender. You could use it in soup, beef pot pie, and so on.