Preheat oven to 400 degrees F (200 C) and set aside a deep 9x13-inch baking dish.
Bring a large pot of water to a rapid boil, then stir in 1 Tablespoon kosher salt followed by the pasta. Cook, stirring occasionally, for just about 5 minutes, until the pasta has begun to soften but is not fully cooked according to package instructions. Drain pasta and set aside.
1 Tablespoon kosher salt, 16 ounces pasta shells
Return the pot to medium heat and add the butter. When butter has melted, add the minced garlic, ground mustard, and cayenne. Cook until fragrant, about 30 seconds. Whisk in the flour and cook, stirring constantly, about 1 minute, until the mixture is thick and the flour takes on a lightly golden color.
6 Tablespoons butter, 3 medium garlic cloves, 1 teaspoon ground mustard powder, 1/4 teaspoon cayenne pepper, 6 Tablespoons all-purpose flour
Pour in the broth a little bit at a time, followed by the milk, whisking well after each addition. Once all the liquid is added and smooth, bring the mixture to a simmer and let it bubble gently for 10-15 minutes, continuing to whisk frequently. Resist the temptation to pour in all the liquid at once or increase the heat to speed this up -- doing either will increase the odds of your sauce becoming grainy. While you wait for the sauce to thicken is a great time to shred the cheese.
2 and 1/4 cups low-sodium chicken or vegetable broth, 3 cups milk
Once the sauce has thickened enough to coat the back of a spoon, turn off the heat. Add the shredded cheese a handful at a time, stirring each addition until melted and smooth. Season the cheese sauce with salt and pepper to taste. (I usually add about 1/2 teaspoon kosher salt and 8-10 cracks of fresh-ground black pepper, but this can vary a lot by family.)
16 ounces cheddar cheese, 8 ounces Colby Jack cheese, black pepper
Mix the drained pasta into the sauce, stirring to break up any clumps. Pour mixture into the baking dish and sprinkle the top evenly with breadcrumbs.
1 cup Italian breadcrumbs
Bake uncovered for 25-35 minutes, until the sauce is bubbling and the breadcrumbs are crisp. For a deeper golden effect on the top, broil for 1-2 minutes at the end. Remove from the oven and sprinkle with parsley or other herbs as desired. Allow to cool for 4-5 minutes, serve and enjoy!