Directions
Step 1: Cook the spaghetti
Preheat the oven to 350°F. Cook the spaghetti according to the package directions. Drain the spaghetti, reserving about two cups of the pasta water so you can mix some into the sauce.
Step 2: Cook the aromatics
Meanwhile, in a Dutch oven, melt the butter over medium heat. Add the onion; cook and stir until the onion is translucent and beginning to soften, six to eight minutes. Add the garlic; cook and stir one minute longer.
Step 3: Make a roux
Whisk the flour into the garlic and onion mixture until blended. Slowly add the milk, whisking as you go. Bring to a simmer and cook, stirring, until thickened, about two minutes.
Step 4: Build the casserole
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