Preheat oven to 200C fan-forced (220C conventional). Place the cauliflower, oil, salt and pepper in 2 x 2.5-cup capacity baking dishes and toss to combine. Loosely cover with foil and roast for 15 minutes.
Meanwhile, mix together mascarpone, cheeses and mustard. Remove cauliflower form the oven add lasagne sheets, placing around the cauliflower, and spoon over the mascarpone mixture.
Return to the oven and bake for 15 minutes, or until golden and bubbling, adding the sage leaves in the last minute of cooking.
Serve with grated parmesan.
