Preheat oven to 425 degrees F.
Line large baking sheet with aluminum foil.
Toss butternut squash in olive oil, then season with salt and pepper, and arrange onto prepared baking sheet.
Roast in preheated oven for 20-25 minutes, until squash is tender.
Bring large pot of water to boil and cook shells al dente (so about 2-3 minutes less than package directions)
While water is boiling, you can brown the butter. Do this in a small saucepan over medium heat, stirring constantly until butter turns an amber color. Once it is amber, remove from heat and add sage leaves and juice from the lemon. Let cool.
In large bowl, add butternut squash and mash with potato masher or you can puree with immersion blender to obtain a smooth consistency.
You should get about 2 cups of squash puree.
If you get more than 2 cups, you can put the extra in a plastic baggie and freeze for later use.
To the bowl with mashed butternut squash, add ricotta, 1/2 cup of Parmesan cheese, garlic, spinach, egg, 1 tsp of lemon zest, and salt and pepper.
Now you are ready to assemble the shells.
Butter a 9x13" casserole dish.
Preheat the oven to 350 degrees.
Stuff the shells carefully with about 2-3 tbsp of the squash mixture.
Don't get too caught up with exact amounts, just do about a heaping spoonful into each shell.
I tried to stuff as much filling as I could into each shell because the filling is so good!
Once shells are all filled and arranged in casserole dish, drizzle half of the brown butter over them, then sprinkle with remaining 1/2 cup of Parmesan cheese.
Bake in preheated oven for 20-25 minutes, until the shells are heated through and the cheese is melted.
Once done baking, drizzle with remaining brown butter.