Wash the savoy cabbage, halve lengthwise and finely slice.
Peel the onions. Halve 1 onion and slice into rings.
Heat 100ml of stock in a pot, add the thyme, cabbage and onions strips and bring to the boil. Cook for 15 minutes over a low heat.
Meanwhile, wash the tomatoes, remove the stalks and slice them.
Dice the remaining onion finely. Peel and finely chop the garlic.
Heat 1 tbsp olive oil in a pan and fry the onions and garlic over a low heat for 5 minutes.
Add the passata, bring to the boil and season with salt, pepper and chilli powder. Cool slightly.
In another pan, bring the rest of the stock and the remaining oil to the boil.
Mix the milk and flour in a bowl and carefully add to the oil-stock mixture, stirring constantly. Bring to the boil again.
Add 2 tbsp of cheese and melt over a low heat. Season the cheese sauce with salt, pepper and nutmeg.
Pour half of the tomato sauce into a casserole (about 20 x 28cm). Lay one layer of lasagne sheets on top, add one third of the cheese sauce and half of the savoy cabbage.
Add another layer of lasagne sheets and the remaining tomato sauce. Add 1 more layer of lasagna sheets, a third of the cheese sauce and the remaining savoy cabbage.
Cover with a final layer of lasagne sheets and the remaining cheese sauce. Arrange tomato slices on top and sprinkle with the remaining cheese. Season with pepper and bake in the preheated oven at 200°C (180°C fan, gas 6).