First slicing, then cooking, or the other way around?

I would like to try out a recipe for a gratin of beets and potatoes. For this I will need sliced potatoes and beets. I always like to cook vegetables for a bit before putting them in the oven to make sure they're done.

Should I first slice the beets and potatoes and then cook them, or the other way around? On the one hand, first slicing feels kind of weird. On the other hand, I'm worried that after cooking, they will be too soft to slice properly. I can't cut the beets in small pieces before cooking (because then the slices will be too small), so the inside will be much harder than the outside as well.

Is there a clever way to do this?