Roast the Squash: Preheat your oven to 425°F / 220°C. Lightly grease a large 15x10x1-inch baking pan. Halve the squash lengthwise, then remove the seeds and peel the tough outer skin.
Cut the squash into ¼- to ½-inch slices and place them in the prepared pan.
Drizzle with olive oil and ½ teaspoon of salt, tossing gently to coat everything evenly.
Spread in a single layer and bake for 25 to 30 minutes, stirring once halfway through, until the squash is tender and lightly caramelized.
Remove from the oven and reduce the oven temperature to 375°F / 190°C.
Create the Creamy Sauce: While the squash roasts, melt the butter in a large saucepan over medium heat.
Add the minced garlic and cook for 1 minute until fragrant, stirring constantly.
Whisk in the flour and the remaining ½ teaspoon of salt to form a paste. Gradually pour in the milk, whisking continuously to prevent lumps.
Continue to cook and stir until the sauce thickens and begins to bubble.
Remove from heat and stir in the roasted butternut squash and fresh rosemary.
Assemble the Lasagna: Lightly grease a 3-quart (or 9x13-inch) rectangular baking dish.
Spread about 1 cup (240 ml) of the butternut squash sauce evenly across the bottom.
Arrange a layer of 3 no-boil noodles on top.
Spread one-third of the remaining sauce over the noodles, then sprinkle with ⅓ cup of Parmesan cheese.
Repeat these layers (noodles, sauce, cheese) two more times. Gently pour the heavy whipping cream evenly over the entire top layer, then sprinkle with the final ⅓ cup of Parmesan cheese.
Bake to Golden Perfection: Cover the baking dish tightly with foil and bake for 40 minutes.
Remove the foil and continue to bake for about 10 more minutes, or until the edges are bubbling and the top is a beautiful light golden brown.
Let the lasagna stand for 10 minutes before slicing and serving. This allows it to set up for perfect, clean slices.
