Preheat oven to 180C. To make the tomato sauce, heat a large saucepan over a medium/high heat and fry the onions in the oil until they start to soften and turn tran-slucent. Add the garlic, chopped tomatoes, passata and lentils, water, salt and pepper and simmer for 20–30 minutes until the sauce thickens and the lentils are tender.
White sauce: heat the olive oil over a low heat in a saucepan and add the flour, stirring to form a paste. Cook, stirring for 1 minute then slowly add the milk. Once all the milk is incorporated, add the mustard and nutritional yeast and heat over a low heat, stirring constantly, until thickened. Separate 1/3 of the sauce into a clean bowl and leave to cool.
Add the wilted spinach to the remaining white sauce and season with grated nutmeg.
Next, fry the mushrooms in a large frying pan with the oil for about 5 minutes, until there is no moisture left in the pan. Add the garlic and thyme and cook for 1 - 2 minutes until everything is fragrant. Season and remove from the heat.
In a bowl, mix the breadcrumbs, garlic, parsley and olive oil together for the topping.
In a medium-sized oven dish, spread some tomato sauce on the base and then lay sheets of butternut squash to cover, top with half the mushroom mixture, and another layer of butternut squash and then half the spinach and white sauce mixture and repeat, layering until everything is used up. Finish by spreading the plain white sauce on top of the lasagne and finally sprinkle with the breadcrumb mix.
Bake for 30–40 minutes, until the top is golden and a knife slides through the lasagne with no resistance.