Preheat oven to 400 degrees F. Spray a baking sheet with non-stick spray and set aside.
Toss delicata squash with olive oil and spices. Place on baking sheet and roast for 25 minutes or until tender.
Transfer squash to a medium sized bowl. Add yogurt, brown sugar and Parmesan cheese. Whip together with a hand blender until creamy. Mix in eggs, and beat for another 4 minutes. Pour mixture into a greased casserole dish. Set aside.
Lower oven to 350 degrees F.
In a heavy bottom light colored skillet, heat the butter gently over low heat until it has melted completely. With a rubber spatula, scrap the brown bits from the bottom of the skillet. Once it smells nutty and toasty, add sage leaves. Cook for another 2 minutes, swirling and titling the pan.
Pour the butter on top of the squash casserole. Arrange leaves like so on top. Bake for 30 minutes or until a tester inserted in the middle comes out clean. Serve.