Preheat the oven to 220C/425F/Gas 7.
Put the kettle on to boil for the vegetables.
Meanwhile, finely grate the parmesan for the topping, toss in a small bowl with the breadcrumbs and set aside.
When the kettle has boiled, tip the prepared sweet potato and butternut squash into a large pan, add a little salt and then pour the boiled water over to cover. Put the lid on, bring back to the boil and then leave to bubble away for 15-20 minutes until tender.
Fill the kettle up with water again and pop it back on to boil while you quickly prepare the herbs for later. Run your fingers down the length of the rosemary stalks to release the leaves and roughly chop them with the sage leaves and set aside in a small bowl.
Pour the boiled water into a large pan, add a drizzle of oil, slip in the lasagne sheets and leave to cook for five minutes. (As there is not loads of liquid in this dish the lasagne sheets do need a bit of precooking).
In the meantime, prepare the sauce. Finely grate the parmesan and peel and grate the ginger. Place both into a medium bowl, add the ricotta, egg yolks, pinch of nutmeg, salt and pepper, stir to combine and set aside.
Drain the lasagne sheets once they’ve had their time and tip them back into the pan. Drizzle in a little more oil and gently toss them about to coat, so that the sheets don’t stick together.
Check that the vegetables are now nice and tender. Drain well and tip them back into the pan. Mash them with a masher until smooth and add salt and pepper to taste.
Now assemble the lasagne in a baking dish, set on a baking tray. Spread a third of the vegetable mash in the base of the dish. Next, scatter a third of the herbs, pine nuts and spinach leaves over. Then arrange four lasagne sheets on top, cutting to fit if necessary. Spread a third of the white sauce over.
Repeat this two more times giving you three layers and finally sprinkle the parmesan breadcrumbs on top. Bake for 15-20 minutes.
Prepare the salad dressing by simply mixing the olive oil, balsamic vinegar, mustard powder (if using) and seasoning together in a small bowl.
When the lasagne is ready, the pasta will feel tender when pierced through with a knife and the top will be crisp and golden-brown.
Cut it into portions, carefully lift out and serve with a handful of rocket drizzled with the dressing.