1. Preheat the oven to 425 degree F.
2. In an oven safe dish, lay chopped squash, and drizzle olive oil on it. Pop it in oven for 35 minutes or until nice and soft.
3. For filling, in a large mixing bowl, combine ricotta, egg, parmesan cheese, garlic, spinach, salt, and pepper.
4. Add lemon zest, and mix it well.
5. Drop cooked squash into ricotta mixture. Gently mix it well.
6. For topping sauce, in a pan, melt butter until brown and bubbly.
7. Drop sage leaves in it. Stir well.
8. Turn off the heat, and pour lemon juice in it. Mix well.
9. Gently fill mixture into the shells. Fill it to the top, and do not over stuff.
10. Place it on a baking sheet, and sprinkle Parmesan cheese on top.
11. Pour butter sauce over it. Sprinkle some fresh sage leaves.
13. Pop baking sheet into 425 degree F oven, and bake for 20 minutes or until slightly brown on top.
14. Serve butternut squash stuffed shells hot with a glass of wine.