Rinse the lentils, cover with 750 ml (3 cups) of water, bring to a boil over high heat and simmer over medium-low until tender, about 20 minutes. Drain and stir into the marinara with the Italian seasoning.
Preheat the oven to 190°C (375°F) with a rack in the centre.
Blend the tofu with the lemon juice, nutritional yeast, garlic, oil and salt until it looks like ricotta. Chop the spinach and fold it in.
Spread a thin layer of sauce in a 23 x 33 cm (9 x 13 inch) baking dish. Add a layer of lasagna sheets, a third of the tofu ricotta and a quarter of the sauce. Repeat twice more.
Finish with a layer of pasta and the rest of the sauce, making sure the sheets are completely covered so they soften.
Cover tightly with foil and bake for 40 minutes, then uncover and bake 10 minutes more, until bubbling and the pasta is tender when pierced with a knife.
Rest 15 minutes before cutting into 8 pieces so the layers hold.