To Make The Stuffed Shells:

1. Preheat oven to 425 degrees F.  Line a large baking sheet with parchment paper or aluminum foil (greased with cooking spray).
2. In a small bowl, stir together the Panko, melted butter, Italian seasoning and 1/4 teaspoon salt until combined.  Set aside.
3. In a large mixing bowl, toss together butternut squash, garlic cloves and olive oil until evenly coated.  Spread the butternut squash out on the prepared baking sheet, and season generously with a few pinches of salt and pepper.  Then remove the garlic cloves and seal them in a small pouch made of aluminum foil, and set the pouch on the baking sheet next to the squash.  Bake for 20 minutes, or until the butternut squash mash easily with a fork and the garlic is soft and fragrant.  Transfer the squash and garlic back to the large mixing bowl along with 1 cup of alfredo sauce, and mash together with a fork until smooth.
4. While the butternut squash are roasting, bring a large stockpot of generously-salted water to a boil over high heat.  (*See tip below about starting the alfredo sauce at this point.)  Add the jumbo shells and cook al dente (or ideally cook until just a minute before they reach al dente) according to package instructions.  Drain.
5. In a 9×13-inch baking dish (or I used an oval dish), spread out 1/2 cup of the remaining alfredo sauce in an even layer.  Stuff a tablespoon or two of the butternut filling inside each of the shells, and lay them in an even layer in the baking dish, filling-side-up.  Spoon the remaining alfredo sauce evenly on top.  (Optional: if using the mozzarella, sprinkle it on top of the alfredo sauce.)  Then sprinkle the Panko mixture evenly over the top of everything.
6. Bake uncovered for 15 minutes, or until the Panko mixture is crispy and starts to turn slightly golden.  Remove and serve immediately.

To Make The Alfredo Sauce:

1. Heat olive oil in a large saute pan over medium-high heat. Add garlic and saute one minute, stirring occasionally, until fragrant. Sprinkle with flour, and stir to combine. Saute for an additional minute to cook the flour, stirring occasionally.
2. Slowly add chicken broth, whisking to combine until smooth. Whisk in milk and sage, and bring the mixture to a simmer. Let cook for an additional minute until thickened, then stir in Parmesan cheese, salt and pepper until the cheese melted.  Reduce heat to low and simmer until ready to use.
3. (*For timing, I recommend making the alfredo sauce simultaneously with bringing the pasta water to a boil and cooking the pasta.)