Heat the oven to 400 degrees F and put a large pot of well-salted water on to boil over high heat.

Meanwhile, in a 5- to 6-quart Dutch oven or other heavy-duty pot, melt the butter over medium heat. Add the onion and 1/2 teaspoon salt and cook, stirring occasionally with a wooden spoon, until softened, 4 to 5 minutes. Add the flour and cook, stirring, until slightly darker, 1 to 2 minutes. Stir in the mustard.

Switch to a whisk and gradually add the milk, whisking constantly. To avoid lumps, go slowly at first, whisking until the mixture is smooth before adding more milk. Once about half the milk is in and the mixture still isn't very thick, you can add the rest more quickly.

When all the milk is in, switch back to the spoon and stir in the thyme sprig, bay leaf, and 1/2 teaspoon salt. Let come to a bare simmer, and cook, stirring frequently, for 15 minutes to meld the flavors (reduce the heat to medium low or low as needed to maintain the bare simmer).

Discard the thyme sprig and bay leaf. Add the Cheddar and Jack cheeses, stirring until melted, and then add the Worcestershire and Tabasco. Season to taste with salt and pepper. Keep warm, stirring occasionally.

Cook the pasta in the boiling water until al dente. Pour into a colander and shake it a few times to drain really well. Add the pasta to the cheese sauce, and stir until well combined. Generously season to taste with salt and pepper. Lightly oil a 9x13-inch baking dish and spread the pasta in the dish.

In a medium bowl, toss the breadcrumbs, Parmigiano, olive oil, chopped thyme, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Scatter the crumbs evenly over the pasta.

Bake in the center of the oven until the crumb topping is golden, about 15 minutes. Let rest for 5 to 10 minutes before serving.

Add flavor or heat with these add-ins:

Pancetta or bacon: Saute 1/4 pound pancetta or bacon (finely chopped if using pancetta) until crisp and golden. Drain on paper towels, leaving the fat in the pan. Crumble the bacon. Substitute the fat for part of the butter so that you have about 6 tablespoons total, and proceed with the recipe. Add the pancetta or bacon to the sauce along with the pasta.

Ham: Brown 2 cups of small-diced smoked ham in the butter over medium-high heat. Drain on paper towels and proceed with the recipe, reducing the heat to medium to cook the onion. Add the ham to the sauce along with the pasta.

Hot chiles: Cook 2 tablespoons minced fresh serrano chiles along with the onions. Or add 1/4 to 1/2 teaspoon crushed red pepper flakes with the mustard.