Are you talking about the right side of the fireplace where it juts out? If so, a few things. The more expensive and nicest looking option is to frame out the left side of the fireplace so it is even with the right. Patch and paint and then mount your TV. The cheapest option would probably be a full motion mount allowing the TV to see in front of both walls. The only downside with that as you couldn't push it back flush against the wall. You could try what you suggested but I have rarely seen it look acceptable to clients when finished.