Gather and prep all ingredients and preheat your oven to 400°F (200°C).
Cut your butternut squash in half lengthwise and brush cut sides with olive oil, sprinkle with ½ tsp salt, ¼ tsp pepper, and 1 tsp poultry seasoning. Place cut-side down on a parchment-lined baking sheet and roast for 40-50 minutes, until tender.
While the squash is roasting, in a large skillet, oil over medium heat. Sauté onion, garlic and mushrooms until softened, and mushrooms have released their liquid and browned about 8-10 minutes.
Add kale and cook until wilted and tender. Season with ½ tsp salt and ¼ tsp black pepper. Set aside.
Scoop squash into a blender or food processor. Add cottage cheese and vegetable broth. Blend until smooth.
Melt the butter in a small saucepan over medium heat. Add the sage leaves and swirl the pan as the butter foams and turns golden brown. Watch closely-the aroma should be nutty and toasty. Remove from heat, let cool for a few minutes, then transfer the crispy sage to a paper towel and set aside.
Return the browned butter to medium heat and whisk in the flour, cooking for about a minute to get rid of any raw flavor. Slowly pour in the milk while whisking constantly, then bring the mixture to a gentle simmer. Keep whisking until it thickens, about 5-6 minutes (see notes).
Stir in Parmesan and 1 tsp poultry seasoning into the bechamel. Taste and season with salt and pepper. Keep warm.
Assemble the butternut squash lasagna: Lightly grease a 9×13 baking dish and spread a thin layer of béchamel on the bottom to prevent sticking. Layer lasagna noodles over the béchamel. Spread a generous layer of squash purée on top. Then add a layer of mushroom-kale mixture, then drizzle with béchamel and sprinkle mozzarella.
Then repeat layering: noodles, squash purée, vegetable mixture, béchamel, mozzarella.
Finish with a final layer of noodles, béchamel, and mozzarella.
Cover with foil and bake at 375°F for 30 minutes. Remove foil and bake for another 10-15 minutes until the top is golden and bubbly. Let rest at least 10 minutes before slicing.
Slice the lasagna and serve, topping each portion with the crispy sage leaves.
*Poultry seasoning is a dried herb blend usually made with sage, thyme, rosemary, and other savory herbs. Any brand will work here, or you can make your own poultry seasoning if you already have the individual seasonings on hand!**I like Better Than Bouillon to make my broths, but you can substitute any vegetable broth.***Frozen kale also works well here. Use 2 cups frozen kale, thaw it completely, then squeeze out as much excess moisture as possible before adding it to the filling.****I use oven-ready lasagna noodles, so there’s no need to boil or par-cook them before layering. If you only have regular lasagna noodles, cook them in boiling water according to the package directions before assembling this lasagna.
Be patient with the béchamel! It may seem thin at first, but it will thicken quickly. Don’t overcook, as it will continue to thicken while the lasagna is assembled. If it gets too thick, stir in a splash of milk until it reaches a pourable consistency.
Prep ahead: For easier prep, you can make the squash purée a day ahead and store it in the fridge. You can also fully assemble the lasagna ahead and refrigerate it overnight before baking. Add a few minutes to the bake time to ensure it heats through fully.Total yield may vary depending on your baking dish. A 9×13 dish usually gives 9 slices, but with straight sides you could get about 12 slices. You may need extra lasagna noodles.