Pre-heat oven to 190°C ( 170°C fan-forced). Combine lentils, stock, pumpkin and bay leaf in a large saucepan. Bring to the boil. Reduce heat and simmer for 15 minutes.
Heat oil in a large frying pan, add onion and garlic. Cook for several minutes until tender. Add lentil mixture to pan with the tomatoes, passata, balsamic and parsley. Simmer for 10 minutes.
Meanwhile, heat butter in a small saucepan, add flour and mix in well. Add milk and whisk until all combined. Bring to the boil and continue stirring until sauce has thickened. Season well.
To assemble, spoon 1/3 of lentil mixture over the base of a 6cm deep, 30 x 22cm baking dish. Top with a layer of lasagne sheets to fit. Top with another 1/3 of the lentil mix, followed by half of the spinach and mozzarella. Cover with a layer of lasagne sheets.
Spread remaining lentil mix into dish, followed by remaining spinach and mozzarella. Lay a double layer of lasagne sheets on the top and then cover with the bechamel. Sprinkle with parmesan and bake for 40 minutes, or until lasagne is tender and top is golden.