Have you tried putting this recipe together (including the potato mixture) a day ahead, or morning of, and then baking right before an event?

I haven't tried making the whole thing ahead of time (I'm sure it would be fine, I just can't guarantee!), but I know if you made the filling and the topping ahead of time and keep them stored separately in the fridge, then assemble right before baking you'll be fine!

Not this shepherd's pie, but yes other similar.
Can be made ahead and refrigerated (unbaked) for maximum 2 days.
Or kept frozen (baked or unbaked) up to a month.