For the Shells:

1. Preheat the oven to 425°F and line a baking sheet with tin foil. Spread the squash, onion, and garlic cloves out in an even layer on the baking sheet and sprinkle everything with the brown sugar. Drizzle with olive oil and season with salt and pepper. Toss everything with your hands to evenly coat. Roast for 20-25 minutes or until the squash is tender and caramelized in spots. Reduce the oven temperature to 350°F.
2. Meanwhile, cook the pasta shells in salted boiling water for 5-6 minutes. You want them to be only slightly tender. Drain and place the cooked shells on a lightly oiled baking sheet.
3. Allow the squash mixture to cool slightly before placing it in the bowl of a food processor. Pulse the machine until the veggies are mostly pureed. Spoon out roughly 1 cup of the mixture and set aside (this will be added to the sauce). Add the ricotta, 1 cup of the mozzarella, half of the asiago cheese, half of the parmesan cheese, and the chopped sage to the remaining squash puree in the food processor. Pulse to combine everything.

For the Sauce:

1. Melt the butter in a medium pot and once melted, whisk in the flour. Cook for a minute or so and then slowly stream in the milk, whisking constantly. Cook over medium heat, whisking almost constantly, until the mixture thickens enough to coat the back of a spoon. Whisk in the reserved butternut squash puree. Add in the cheeses and the sage and stir until everything is smooth.

To Assemble:

1. Pour half of the sauce into the bottom of a buttered 9 x 13-inch baking dish. Spoon the filling into the cooked shells and arrange them in the sauce layer.
2. Once all the shells have been filled and placed in the dish, pour the remaining sauce over top and top with the remaining mozzarella, asiago, and parmesan cheeses. Top with whole sage leaves, if desired.
3. Cover the pan with tin foil and bake for 20 minutes. Remove the foil and bake for another 10-15 minutes or until the top is golden brown and the sauce is bubbling.