For the lentil layers, heat the oil in a medium, heavy-based saucepan. Add the vegetables to the pan. Season and cook gently for several minutes until they begin to soften. Add the rosemary and cook for a further 5 minutes.
Add the lentils, stir to combine and cover with cold water. Crumble the
stock cube into the pan, bring to a gentle simmer and stir frequently to
prevent the lentils from sticking. Once most of the liquid has been absorbed, top up with water until the lentils are cooked; they should be soft but not mushy – this should take around 15–20 minutes. Set aside.
Preheat the oven to 180˚C/gas mark 4. Bring a large pan of water to the boil and blanch the spinach for several minutes until it is just wilted. Remove with a slotted spoon and press firmly to release any excess water.
In the same pan, add the olive oil before dropping in the lasagne sheets.
Cook the sheets until they are al dente, but be careful not to overcook. Drain, rinse with cold water and set aside until needed.
For the sauce, melt the butter in a pan and add the flour, stirring until it
thickens. Add half the soya milk, stirring constantly until smooth. Season, add the nutmeg, mustard, vinegar and remaining milk. Cook until smooth and glossy, stirring occasionally – add a little more milk or water if it becomes too thick. Check the seasoning and then assemble the lasagne.
Arrange the spinach in the base of a lightly greased 19cm baking dish.
Season with pepper and nutmeg and pour over one-third of the béchamel
sauce. Place three lasagne sheets on top, then half the lentil mixture, a further three lasagne sheets, the remaining lentils and finally the last sheets of pasta.
Pour over the rest of the béchamel and a generous grating of nutmeg.
Bake for 20–25 minutes until the top begins to bubble and turns golden.
Allow to cool for 5 minutes before serving.