Read the lasagne-sheet instructions first; pre-cook if the pack requires it. Heat a conventional oven to 180 °C.
Soften onion and carrot in oil for 8–10 minutes. Add courgettes and cook for another 5 minutes. Add tomatoes and oregano, then simmer for 15 minutes. Season.
Make the béchamel. Layer a little sauce, pasta sheets, vegetables and béchamel in a baking dish. Finish with béchamel.
Cover for the first 25 minutes, then uncover and continue until the pasta is tender and the centre is hot. Total time depends on the sheets. Rest for 10 minutes before serving.
Check that all packaged ingredients are suitable for your dietary needs. Lactose-free dairy still contains milk proteins; use suitable plant-based alternatives if you need to avoid dairy.