In honor of Pi Day... low cal recipes?

I post here with one request: give me your favorite low-calorie pie recipes! Cherry preferred, but I can adapt anything

It's Pi Day, so I'm making pie tonight to celebrate with my boyfriend and roommates! I don't want it to be "topless" - I'm okay with some sort of crumb topping, though, rather than an entire pie crust on top.

I've searched and searched and just can't seem to find a pie recipe that's very low-calore (the crust seems to be the main issue). It may be a lost cause, but I thought I'd ask you guys just in case!

Happy Pi Day everyone!