What does in true fashion mean?

It means that you're doing something in a way that is true to its form.

Let me provide an example. Let's say it's Super Bowl Sunday. I could say...

In true Super Bowl fashion, I'm going to have all of my friends over to watch the game.

The point of saying "in true fashion" is to IMPLY that everything associated with whatever we're referring to, Super Bowl Sunday in our case, is applicable to the situation.

Another example:

In true underdog fashion, the team came back after halftime and won the game.