Preheat oven to 400 degrees.

Cook macaroni until al dente, then drain and rinse with cold water.

Cut butternut squash in half lengthwise, drizzle with olive oil, and place cut-side up on a baking sheet.

Roast squash for 20-25 minutes, or until fork tender. Watch to prevent burning.

Scrape out squash flesh and mash with a potato masher. Season with salt and pepper.

Slice yellow onions.

In a large, ovenproof skillet, cook onions with 2 tablespoons of butter over low to medium-low heat until deep golden brown. Remove and set aside.

Melt 4 tablespoons of butter in the same skillet over medium-low heat.

Sprinkle in flour and whisk to make a thin paste. Cook for 2 minutes.

Whisk in milk and cook, stirring gently, for a few minutes until slightly thickened.

Season the sauce with salt and pepper to taste.

Stir in mashed butternut squash until warmed through.

Stir in grated cheddar cheese. Add a splash of milk if the sauce is too thick.

Keep stirring until the sauce is hot, then stir in the cooked macaroni.

Season with additional salt to taste.

Melt the remaining butter and combine with breadcrumbs.

Sprinkle breadcrumbs over the macaroni mixture.

Bake in the skillet at 400 degrees for 15 minutes, or until crumbs are golden and the edges are bubbly.

Serve immediately.