Peel the butternut squash thoroughly and slice it into 1/8-inch thick half-rounds. Remove and discard the seeds. Set the prepared squash aside for layering later. If you have extra squash from the bottom, you can save it for another use.

In a small saucepan, melt the butter over medium heat. Add the finely minced shallot and coarsely chopped garlic, cooking until fragrant for about 1 to 2 minutes. Sprinkle in the all-purpose flour and whisk continuously with a wooden spoon to form a roux, letting the mixture thicken. Gradually pour in the low-sodium vegetable broth, whisking constantly to prevent lumps. Add in the heavy cream, black pepper, onion powder (if using), and whole sage leaves. Stir and heat gently until smooth; do not let it come to a boil.

Preheat your oven to 400ºF (200ºC). Use a small knob of butter to grease a 9 x 13-inch baking dish well. Pour one-third of the prepared cream sauce from Step 2 into the bottom of the greased baking dish. Sprinkle a small amount of the grated parmesan cheese over the sauce. Arrange the butternut squash slices (from Step 1) in two or three overlapping rows on top of the sauce and cheese. I like to overlap the squash for an attractive, rustic look. Drizzle the remaining two-thirds of the cream sauce evenly over the squash layers.

Cover the baking dish with a sheet of parchment paper placed directly onto the gratin's surface, then wrap with aluminum foil on top to prevent the foil from touching the food. Place the covered baking dish in the preheated oven and bake for 20 minutes. After this initial bake, remove both the foil and parchment paper, then sprinkle the remaining parmesan cheese evenly over the gratin. Return the dish to the oven and bake uncovered for another 10 minutes, or until the top is golden and crispy.

Take the finished gratin out of the oven and let it cool slightly for a couple of minutes. Sprinkle with the chopped fresh chives for a pop of color and mild onion flavor, if desired. Serve the gratin immediately while it's warm and bubbly. For a bit of extra richness, I sometimes drizzle a small amount of browned butter over the top before serving.