Yeah I've gone wrong (had big old binge last week) but I still lost weight because I got straight back on the wagon the next day. Don't look back & wrack yourself with guilt about one mistake. Draw a line under it & move on:
Drawing a line
Think about recording how you felt when you ate the lasagne & how you feel now? It could be useful for the future so you can refer back if you feel tempted to go wrong again. Remember if you could control yourself around food you wouldn't need cd would you? See this as as a lesson learned!
Move on
Drink lots of water, keep yourself busy, be good for the rest of the week, get weighed and see how you've done. Chin up chuck, you can do it!