Sloppy vegan cheese and onion pie. Where am I going wrong?

[FireMeetGasoline]

Hey, I wonder if anyone can help? I made a vegan cheese and onion pie tonight, and it tasted delicious. The problem being, when I scooped the pie out of the dish, it was just a sloppy mess with a pastry hat. Am I boiling the potatoes for too long? The filling would have made a great pasty, and would actually rival a Gregg's cheese and onion, but as a pie, it isn't getting the firmness I would like. I boiled the potatoes so I could mash them, added butter, mustard, the cheese and raw onion. Tasty, but very runny 😂. Please help!

I would add a cornflour slurry to thicken it up

Chuck a handful of breadcrumbs in the mix before you bake it

@Mandofan great suggestion, thanks. I never thought of that because whilst I add it to thicken up sauces, the consistency of the mashed cheese and potato didn't look like it needed it. I will definitely do this next time

Oh, @sadpapercourtesan, another great suggestion. Thanks to you both.