Preheat the oven to 425°F. Lightly butter a 3-quart au gratin or baking dish; set aside. Cook pasta according to package directions. Drain; transfer to a large bowl.

Meanwhile, in a large saucepan combine the squash and 2 1/2 cups of the milk over medium-high heat. Bring to boiling; reduce heat to medium, and simmer until the squash is tender when pierced with a fork, 18 to 20 minutes. Stir together remaining 1/4 cup milk and flour; stir into squash mixture. Bring to boiling; cook until thickened, 2 to 3 minutes. Stir in 1 1/2 cups of the Gruyere until melted; keep warm.

Meanwhile, in another very large skillet cook chopped bacon until crisp; drain on paper towels. Crumble; set aside. Pour off all but 2 tablespoons bacon drippings. Return skillet to the heat.

Add onions to skillet; cover and cook over low heat 10 minutes, stirring occasionally. Uncover and increase heat to high. Cook 4 to 6 minutes more, stirring, until onions are golden. Mix together with cooked bacon and set aside about 1/4 cup.

Add squash-cheese mixture, onions, and bacon to the bowl with the pasta. Toss well to combine, then transfer to prepared baking dish.

Mix panko with melted butter & 1/4 cup bacon/onion mixture. Sprinkle remaining Gruyere and the bread crumbs over pasta mixture. Bake until top is browned, about 10 to 12 minutes. Cool 5 minutes. Sprinkle with parsley if desired. Makes 6 to 8 servings.