Tender strings of spaghetti squash replace pasta in this cheesy gluten-free casserole that's studded with broccoli. Cheddar cheese melts into a bubbling topping. Serve with roasted chicken or pork or as part of a vegetarian dinner.
Preheat oven to 400°F. Lightly coat an 8-inch-square baking dish with cooking spray.
Place squash halves cut-side down in a microwave-safe dish. Add 2 tablespoons water. Microwave on High until the squash is tender, 10 to 15 minutes. (Alternatively, place squash halves cut-side down on a rimmed baking sheet. Bake in a 400°F oven until the squash is tender, 40 to 50 minutes.) When cool enough to handle, use a fork to scrape the squash flesh into a medium bowl. Season with 1/4 teaspoon each salt and pepper.
Meanwhile, heat oil in a large skillet over medium-high heat. Add onion and cook, stirring, until softened and starting to brown, 2 to 3 minutes. Add broccoli and cook, stirring, until tender, 4 to 5 minutes. Add garlic and cook, stirring, for 1 minute.
Whisk milk, cornstarch and the remaining 1/4 teaspoon each salt and pepper in a measuring cup. Add to the pan along with 1 1/4 cups cheese. Cook, stirring, until thickened and the cheese is melted, 1 to 2 minutes more. Remove from heat and stir in the squash flesh. Transfer to the prepared baking dish and sprinkle with the remaining 1/4 cup cheese. Bake until heated through and browned in spots, 15 to 18 minutes. Let rest for 10 minutes before serving.