Preheat oven to 400°F. Line a small baking pan with parchment paper and set aside.

In a large bowl, combine squash, olive oil, sage, salt and pepper. Toss until squash evenly coated with seasonings. Spread squash in a single layer on prepared pan. Roast for 20 minutes or until squash is tender. Stir once, halfway through cooking time. Let cool and mash well with a fork or potato masher.

While squash is roasting, make filling and cook shells. In a medium bowl, combine ricotta, Parmesan, spinach, egg, nutmeg and a pinch of salt and pepper. Mix well. Cover and refrigerate until ready to use. Cook shells according to package directions or until al dente. Rinse with cold water and drain well.

Add mashed squash to ricotta filling and mix well. Stuff each shell with about 2 tbsp of filling. Once all shells are stuffed, set them aside and quickly make the sauce.

In a small pot, melt butter over medium heat. When frothy, add garlic. Cook and stir for 1 minute, or until garlic becomes golden.

Whisk together cream and flour in a measuring cup until smooth with no lumps. Add to pot. Cook, whisking constantly, until mixture bubbles and begins to thicken. Add Parmesan, lemon zest, salt and pepper. Whisk until cheese is melted. Remove from heat.

Reduce oven temperature to 375°F. Spray a casserole dish with cooking spray or lightly oil. Spread ½-cup sauce over bottom of dish. Arrange shells over sauce. Drizzle remaining sauce evenly over shells. Cover loosely with foil and bake for 25 minutes. Uncover and bake for 10 more minutes. Cool slightly before serving.