Sautee onion: In a large pan heat 1 tablespoon of the oil, add the garlic, onion and celery and cook gently for about 4 minutes until softened, stirring often. Add the wine and cook for another minute.
Add lentils: Add the lentils and stock, stir, cover, bring to the boil then simmer for about 12 minutes or until all the moisture has been absorbed and the green lentils are cooked.
Add vegetables: Stir in the zucchini, pepper, herbs, balsamic vinegar and passata. Cover and continue cooking gently for 10-12 minutes, stirring occasionally. In the meantime prepare the mushrooms.
Fry mushrooms: In a large pan heat 1 tablespoon of the oil, add the mushrooms and cook over a fairly high heat for about 6 minutes or until the moisture has been absorbed and the mushrooms are lightly browned, stirring occasionally. Add the soy sauce and cook for a few more seconds. Stir the mushrooms into the sauce and adjust the seasoning as needed.
Make sauce
Make roux: In a saucepan melt the butter, add the flour and whisk until smooth.
Add milk: Pour in some of the milk, increase the heat and whisk until the sauce thickens. Add the rest of the milk and continue cooking until the sauce starts bubbling up. Remove from the heat, season lightly and set aside for 5 minutes.
Assemble
Preheat the oven to 375F/ 190C/ gas mark 5. Lightly grease the bottom of your lasagna dish (20x28 cm/8x11in) and cover with 3 lasagne sheets.
Add filling: Spread half of the filling mixture over the top then cover with 3 more lasagne sheets. Add the second layer of the filling mixture.
Make ricotta layer: In a bowl combine the ricotta with 6-7 tablespoons of the white sauce, 5-6 tablespoons of finely grated parmesan, nutmeg and salt and pepper to taste. Stir thoroughly and spread this mixture over the lentil sauce.
Add white sauce: Cover with the 3 remaining lasagne sheets and spoon the rest of the bechamel over the top ensuring it covers the pasta sheets completely. Scatter the rest of the parmesan (coarsely grated) over the whole thing.
Bake in the centre of the oven for 40-45 minutes or until golden-brown on top. If the top starts browning too much cover loosely with tin foil after 30-35 minutes. Remove from the oven, set aside for 10 minutes then serve.
Notes
The filling: Taste the sauce before assembling the lasagne to ensure it is well seasoned.
Cheese: I used a moderate amount but you can adjust the amount according to your preference.
Bechamel sauce: The sauce should be quite thick and cover the top of the lasagna evenly.
Baking: Bake your lasagne with lentils until golden-brown on top. If it starts browning too much cover loosely with foil after about 35 minutes.
Best served hot. Leftovers can be refrigerated, covered, for up to 3 days.
Freeze in an airtight container for up to 3 months.