What can I do to make my Cheese & Onion Pie freeze well?
The way to freeze this well would be to use instant potatoes simply mixed cold and not cooked.  Then when you bring it out to reheat, you'll cook the potatoes.  The taste will be of instant potatoes, though.

The problem is that the starches in the potato will undergo retrogradation, recrystallize and expel moisture when reheated.

You could counteract this by stirring in some potato starch after the potatoes have cooked and cooled and before you assemble the pie, but the instant potato method will work better and will require less experimenting.