How to re-run the "app optimization" process after upgrading to a higher version of Android?

After each OTA Android upgrade, I get a message saying "Optimizing app x of xxx" that takes a while. The last time I upgraded, the battery ran out during the optimization process (not during the upgrade itself, which I know it's dangerous). After rebooting the phone the "optimization" process didn't carry on. So, how can I start that process by itself without upgrading anything? (my Android is already up to date).