Preheat the oven to 400 F.

Start by boiling a pot of water for the pasta on the stove. While it's heating, in a medium saucepan, melt the butter on medium-low.

When melted, whisk in the flour and cook for 1-2 minutes (this is a roux). Whisk in the milk, trying to fully incorporate the roux.

Add the garlic. Cook on medium heat, whisking occasionally, until the sauce has thickened (about 7-10 minutes). Add a sprinkle of salt and pepper. 

When the water is boiling, add about 1 tsp of kosher salt, and add the pasta. Cook according to package directions. When done, drain, and keep in a colander.

While the pasta is boiling, return to the sauce. Over low heat, very slowly whisk the cheddar into the milk sauce, about 1/4 cup at a time. The sauce should continue to look like a sauce, and should not have clumps or be stringy. If it starts to get stringy, slow down! Once the cheese is fully incorporated, season with salt and pepper to taste.

Add the pumpkin to the cheese sauce, and whisk to combine.

In a baking dish large enough to fit the pasta, combine the pasta and sauce. Stir gently to combine well. Sprinkle breadcrumbs over the top.

Bake uncovered for about 20 minutes, or until the breadcrumbs begin to brown and the cheese sauce becomes bubbly.

Serve immediately.