Preheat oven to 425 degrees F. Peel, seed, and chop the butternut squash into ½-inch pieces. Place on a rimmed baking sheet and toss with the olive oil, garlic, Italian seasoning, ½ teaspoon salt, and a few grinds of black pepper. Roast in the oven for 25 minutes, until tender, stirring halfway through. Remove from the oven and lower oven temperature to 350 degrees F.

Meanwhile, bring a large pot of salted water to a boil. Add the pasta shells and cook according to package directions for al dente, about 12 minutes. Drain and lay shells on a sheet of parchment paper to cool.

In a medium saucepan, melt the butter over medium-low heat. Add the sage and continue to cook, stirring constantly, until brown flecks appear in the butter. Be very careful as the butter can go from brown to burned very quickly. Remove the pan from the heat and add the garlic. Slowly whisk in the flour; return the pan to the heat and whisk in the milk. Cook, stirring, until sauce thickens. Remove from the heat and stir in the Parmesan, nutmeg, and salt and pepper to taste.

Scoop the roasted squash into a medium bowl. Use a fork to mash the squash. Add the cottage cheese and ½ cup of the alfredo sauce and stir to combine.

Spray a 9x13-inch baking dish with cooking spray. Scoop about 1 ½ tablespoons of the squash mixture into each shell and place shells, seam side up, in the baking dish. Pour the remaining alfredo sauce over the shells. Sprinkle with the chopped walnuts. Bake, uncovered, for 12-15 minutes until hot. Serve.