Heat the oven to 425 degrees. Spray a 1 1/2-quart shallow casserole with cooking spray.
Place the squash and onions in a 15x10-inch baking pan with sides. Drizzle with oil, stirring to coat all sides. Bake for 25 to 30 minutes or until the squash and onion are tender, stirring twice. Remove from the oven; reduce the heat to 375 degrees.
Meanwhile, in a 1-quart saucepan, mix the milk and flour with a wire whisk until smooth. Whisk in the salt, thyme, pepper and nutmeg. Cook over medium heat until the mixture boils, stirring frequently. Boil for 1 minute. Remove from the heat and stir in the sour cream. Set aside.
In a medium bowl, mix the breadcrumbs, butter, Parmesan cheese, green onions and sun-dried tomatoes.
Spoon the squash and onion into the casserole dish. Pour the sauce over the vegetables and stir to coat. Top the squash with the breadcrumb mixture.
Bake for 20 minutes or until bubbly around the edges and the crumbs are golden brown.