While the squash is roasting, cook the shells 2 minutes less time than on the box instructions.
When the shells are ready, drain them, and put them on a paper towel-lined baking sheet to cool.
Reduce oven heat to 375°.
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.
Prepare a piping bag with a large 1A tip and pipe the spinach ricotta mixture into the shells.
Arrange the filled shells on top of the 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.