Thaw the spinach. Chop the seitan very finely.
Peel the carrots. Wash the celery. Peel the onion and garlic. Cut everything into small dice.
Heat the oil in a pot and fry the vegetables over a medium heat for 3 minutes. Add the chopped seitan and cook for another 3 minutes. Season with salt and pepper.
Add the tomato and stock and simmer over a medium heat for 20 minutes, stirring occasionally. Coarsely crush the fennel seeds and add to the pan. Season with salt and pepper.
Meanwhile, finely grate the Parmesan. Drain the spinach well and cut into small pieces. Mix the spinach with the Parmesan, ricotta, salt, pepper and freshly grated nutmeg.
Cover the bottom of a greased baking dish (about 30 x 20cm) with some sauce. Cover with 4 lasagne sheets.
Spread a third of the spinach mixture over the lasagne sheets. Top with a quarter of the sauce. Layer another 4 lasagne sheets, a third of spinach and a quarter of the sauce on top and repeat once more.
Place the last lasagne sheets on top and finish with the remaining sauce.
Drain the mozzarella, tear into pieces and scatter over the lasagne. Bake in the preheated oven at 180°C (160°C fan, gas 4) for 35-40 minutes. Stand for 5 minutes before serving.