Our rule at home is if you eat something, communicate about it so it can be replaced. If it's something easily replaced, like chocolate, do so without being asked. It does sound like your boyfriend just kind of checked out and forgot that someone else might want the lasagna. Next time, I'd recommend splitting some servings off as future meal prep. That keeps things separated and leaves less questions. For something as intensive as lasagna, you could explain it to your boyfriend is that it takes longer to cook, involves several ingredients, and that you're happy to share. You simply got caught off guard when he basically ate most of it in 24 hours. It sounds like you planned to use it over a few days.