A whole tray of lasagne got burnt. How do I write off a dish, not just an ingredient?

It depends on what you count. If the lasagne is a counted dish, open Menu → Inventory and use Set on its row to write the new lower number. The change is written under Stock movements with the reason adjust, so you can see it later. If you count the ingredients instead, use the Wastage panel on Ingredients & suppliers: pick each ingredient the tray used, type how much and why, and press Record. It comes off the shelf and is written down as waste with your reason. There is no single button for wasting a finished dish's whole recipe today, so a tray with four ingredients is four wastage lines. The Stock, recipes and suppliers tab in the docs has the step-by-step for both.