Arrange the squash slices onto a 18" x 26" baking sheet, drizzle with olive oil and sprinkle generously with salt and pepper. Rub each slice to coat well and place in the preheated oven for 20 minutes.
Remove the pan from the oven, flip the squash slices over and arrange the sliced onions and smashed garlic around the squash; return the pan to the oven for 25 minutes or until the onions and garlic are slightly golden and the squash is tender.
Meanwhile, combine all the ingredients for the vinaigrette in a small bowl or glass measuring cup and whisk vigorously until well combined; reserve.
When the squash is done to your liking, arrange it on a serving plate along with the onions and garlic, drizzle with the vinaigrette and top with the Parmesan shavings and pumpkin seeds.