Preheat oven to 180C. Roast your chunks of squash tossed in oil for about 30-40 minutes, until just shy of being perfectly cooked - you want a little bite left in them to keep their shape, as remember they will cook for longer later. Toss them around half way through. You could do this bit ahead of course.
Wilt the spinach in a hot pan by gently stirring it around over a high heat in just a tiny bit of water - the water that is still clinging to the leaves after you have washed it is just perfect. Do this until it wilts, around a couple of minutes is usually enough. Set aside to let it cool for a minute, then squeeze all the water out - be brutal, you don't want a watery lasagne.
Now make your bchamel by putting all the ingredients - cold (it won't work with hot milk) - into a large jug or bowl and cook for 5-7 minutes on high. This will differ for each microwave as they vary so much. Whisk every minute or so. After a while the butter will melt, the flour lumps dissipate - and then suddenly it will really start to thicken. At this point, turn down the heat to medium and cook out the flour for a further 3 or 4 minutes. Add a spoonful of mustard, some salt and pepper, and set aside.
Once you have your components ready - the roasted squash, wilted and squeezed spinach and bchamel sauce ready - begin to make the lasagne. Start by mixing a couple of ladles worth of sauce into the spinach to make it nice and saucy. Then ladle a little sauce into the base of the dish to prevent the lasagne sticking. Add 3 sheets (or 4 if your baking dish is huge) to the base of the dish. Now spread the spinach and sauce mix into the dish on top of those sheets. And now 3/4 sheets more. Tip half the squash in, along with half the remaining sauce on top. And 3/4 more sheets. The rest of the squash and rest of the sauce. Now crumble the cheese all over and spread the whole thing with a palette knife or similar to smooth.
Bake for 35-45 minutes or until bubbling and brown on top. Enjoy (it's even better reheated the next day).