Preheat the oven to 400 ̊. Drizzle the squash with olive oil and place it cut-side up on a rimmed baking sheet. Roast until tender and browned in spots, 50 to 70 minutes, watching it to make sure the surface doesn’t get burned. Scrape out the flesh and mash it with a potato masher. Sprinkle with salt and pepper and set aside. Leave the oven on. Meanwhile, bring a large pot of salted water to a boil. Add the pasta and cook as the package directs for al dente. Drain, rinse under cold water and set aside. Combine the onions and 2 tablespoons butter in a large cast-iron skillet over low to medium-low heat. Cook, stirring occasionally, until the onions are a deep golden brown, about 20 minutes. Remove to a plate. Add 4 tablespoons butter to the same skillet and melt over medium-low heat. Sprinkle in the flour and whisk to make a thin paste. Cook for 2 minutes, then whisk in the milk. Cook, whisking gently, until slightly thickened, about 2 minutes. Taste and adjust the seasonings. You’ll probably want to add more salt. Turn the heat to low and stir in the mashed butternut squash. When it is warmed through, stir in the cheese. If the sauce is too thick when the cheese is melted, add a splash of milk. Keep stirring until the sauce is nice and hot. Stir in the onions and then the macaroni. Taste and add more salt if needed. Melt the remaining 2 tablespoons butter in the microwave. Add the breadcrumbs and toss to coat. Sprinkle the breadcrumbs over the macaroni and bake it straight in the skillet until the crumbs are golden and the edges are bubbly, about 15 minutes.