Preheat the oven to 350F and line a baking sheet with parchment paper.
In a large mixing bowl, combine the diced onion, turnips, carrots, olive oil and fresh rosemary, and season with salt and pepper to taste.
Arrange the squash halves cut-side-up on the baking sheet. If they are wobbly, you can slice the rounded bottoms off to give them a flat bottom to them stand up straight.
Bake at 350F until the squash is fork-tender, about one hour, then serve warm.
Sprinkle with some cheese or favorite dressing to finish!