01 -
Heat the oven to 190°C / 375°F. Use a baking dish about 20 × 25 cm and at least 6 cm deep. Check the lasagne sheet packet: use sheets that cook in sauce, or pre-cook if instructed. Drain and rinse the lentils.
02 -
Heat oil in a wide pan. Cook onion for five minutes, add garlic for 30 seconds, then stir in passata, stock, oregano, lentils, salt and pepper. Simmer gently for eight minutes. Fold in spinach in batches until wilted. Keep the sauce loose.
03 -
Melt butter in a saucepan. Stir in flour and cook for one minute. Gradually whisk in milk until smooth, then simmer for 3–4 minutes, stirring, until lightly thickened.
04 -
Spread a thin layer of lentil sauce across the dish. Add three lasagne sheets, breaking corners to fit without thick overlaps. Add one-third of the remaining lentil sauce and one-third of the white sauce. Repeat twice more with the remaining sheets and sauces.
05 -
Scatter mozzarella over the top. Cover with lightly oiled foil and bake 30 minutes. Uncover and bake 15 minutes more, until the top is golden and a knife slides easily through the pasta. If the pasta still feels firm, re-cover and bake another 5–10 minutes.
06 -
Rest 15 minutes before cutting into six portions so the layers hold. Serve warm, without additional garnish.
07 -
Cool leftovers within two hours and refrigerate up to three days. Reheat covered until piping hot throughout, adding a spoonful of water if needed.