Heat oven to 180C/160C fan/gas 4. Bring a large pan of salted water to the boil. Drop in the cauliflower florets and cook for 10 mins until soft but holding their shape. Drain and allow to steam-dry.
Meanwhile, heat the oil and butter in a frying pan over a medium heat. Add the onion and a pinch of salt, then cook for 8 mins until softened and caramelising. Stir in the garlic, chopped sage leaves and chopped chestnuts, and cook for 2 mins or until fragrant. Remove the pan from the heat and leave to cool slightly.
Grease and line the base of a 23cm springform cake tin with baking parchment. Put the flour, polenta, baking powder and cheese in a large bowl. Whisk in the eggs so that you have a smooth batter, then carefully fold in the cooked onion mixture and cauliflower.
Tip the mixture into the prepared tin and arrange the remaining sage leaves and chestnuts on top. Bake for 40 mins until golden brown. Serve warm as a main course with a big green salad, if you like.