I have wooden eggs in every nestbox, we collect eggs often (several Xs in the morning hours), and there is a ton of available oyster shell around. Oh, and they get treats.
And I've still had the rare egg-eater. *SIGH* When it happens I collect even more frequently. This has worked (and I can return to regular collecting in a few days).
The one eating the mustard eggs probably thought, "Oh, how nice of my human to bring me condiments." Come on, chickens, get it together!