How to keep fredh
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You can just keep them covered to avoid the top from drying out. Also filling should not be too moist to keep bottoms from becoming soggy.
Sam is a trusted home cook.
If you're using a moist filling. You can coat the bread with a thin layer of soft butter before filling to store them. You might even want to chill that a bit first before adding the filling so the butter sets back up.
Anything more than a couple of hours in advance is gonna be soggy or stale. Just ask your kids how great their lunches are when made the night before.