I recommend starting by boiling your lasagna noodles, according to package instructions and then adding one additional minute to the cooking time. Since we aren't baking this for quite as long as a traditional lasagna, we need to give the noodles a bit of extra love at the start. When they're done cooking, set them aside to cool, rub with a bit of olive oil to prevent sticking, if necessary.

While the lasagna noodles are cooling, using the same boiling water you just used to cook them, boil the squash until it's fork tender, about 6-8 minutes. Drain and put the squash into a large bowl. Add the ricotta and a pinch of salt and pepper. Using a hand masher, mash until the mixture is pretty smooth, but not silky. You'll be adding cream sauce to this later, so don't worry if it's not completely mashed just yet.

Now, make the cheese sauce by heating a large pan over medium heat and melting the butter completely. Once the butter has melted, add the flour, half at a time, whisking to incorporate it completely. At this point, you will have a butter and flour 'paste' that looks a bit like vanilla pudding. Add the stock and bring to a brief simmer. Add the cream and simmer once more, about 15 seconds. Add the cheese, salt, pepper, sage and nutmeg, and stir until the cheese is melted. I've attached a picture of what the finished sauce should look like. At this point, you can lower the heat to the lowest setting just to keep the sauce warm. It's time to assemble!

Preheat an oven to 375 degrees. Add 1/4 cup of the cheese sauce to the bottom of an 8x8 inch baking dish. Add half of the remaining cheese sauce to the mashed butternut squash and stir to combine, mashing with a hand masher a bit more to smooth the squash mixture, if desired.

Lay the lasagna noodles out on a cutting board or counter where they won't stick. Spoon two tablespoons of the squash mixture on to each noodle and spread into an even layer, as shown in the picture below. 

Lay a piece or two of prosciutto on the squash mixture, as shown in the picture below, then roll the lasagna noodle into a pinwheel and place into the baking dish so that it's standing on its end. 

Repeat the process with all 12 noodles. If you're using an 8x8 dish, they should fit snugly in the dish, as shown.

Pour the remaining cheese sauce over the top of the pinwheels and cover the top with the mozzarella cheese. Bake, uncovered, for 30 minutes, until the edges are brown and the cheese is entirely melted. Garnish with the chopped walnuts. Enjoy!