Preheat the oven to 200°C and grease the ovenproof dish with butter.
Soak the lasagne sheets in warm boiled water to soften.
Heat oil in a pan and brown the sausage meat for 5–10 minutes.
Stir in the flour and fry for 1 minute.
Add chilli, garlic, and mushrooms, then fry for 5 minutes.
Stir in the crème fraîche and spinach, boil for 2 minutes, and season.
Mix the passata, tomato paste, sugar, and herbs in a bowl to make the tomato sauce.
Drain the lasagne sheets.
Layer one-third spinach sauce, one-third tomato sauce, and half the pasta sheets.
Repeat layers and finish with remaining sauces and grated cheese.
Bake for 30–35 minutes until golden and bubbling.