While the squash is roasting, cook the shells 1-2 minutes less time than on the box instructions.
When they are finished cooking, rinse them in cool water and set them aside while you make the filling.
Mix the spinach, ricotta cheese, parmesan cheese, and egg. Spray a large casserole dish with cooking spray.
Spoon a layer of squash puree on the bottom of the casserole dish.
Fill the shells with the spinach and cheese mixture and place them on the squash puree.
Top each of the cheese stuffed shells with more squash puree.
To make the brown butter sauce, heat the butter in a small saucepan until it begins to turn brown and foamy.
Remove it from the heat and add the sage leaves. Let it sit for about 5 minutes.
Pour the hot butter mixture over the entire dish.
Zest the lemon over the shells.
Bake covered for 25 minutes.
Uncover it and bake another 5 minutes.
Let sit for 5 minutes before serving.
Squeeze a bit of fresh lemon juice over the shells right before serving.