|Butter (for the pan)|
|Salt, to taste|
|1||pound elbow macaroni|
|¾||pound mixed shredded mild, sharp, and extra-sharp cheddar|
|1||Spanish onion, coarsely chopped|
1. Set the oven at 350 degrees. Generously butter a 9-by-13-inch baking dish.
2. In a large pot of boiling salted water, cook the macaroni, stirring occasionally for 8 minutes or until almost cooked through. Drain into a colander; return to the pot.
3. In a saucepan, melt 2 tablespoons of the butter. Add the flour and whisk constantly until smooth. Add the milk, a little at a time, whisking constantly. When sauce thickens and simmers, remove from the heat. Pour the sauce and cheese into the macaroni and stir well.
4. Transfer the macaroni mixture to the dish and cover with foil. Bake for 25 minutes.
5. Meanwhile, in a skillet over medium-low heat, melt the remaining 2 tablespoons butter. Add the onion and cook, stirring occasionally, for 10 minutes or until the onion caramelizes.
6. Remove the foil from the mac and cheese. Scatter the caramelized onion on top. Return the dish to the oven and bake for 10 minutes more. (Total baking time is 35 minutes.) Adapted from Plymouth Artisan Cheese