Heat oven to 350° F. Butter eight 6-ounce baking dishes or two 8-inch square baking dishes. Cook the pasta for 2 minutes less than the package directions suggest; drain and return it to the pot.
Melt the butter in a large pot over medium heat. Add the flour and cook, stirring, for 2 minutes (do not let darken). Gradually whisk in the milk; add 1¼ teaspoons salt and ¼ teaspoon pepper. Bring to a boil, reduce heat, and simmer, whisking occasionally, until the sauce thickens slightly, 10 to 15 minutes. Remove from heat and gradually whisk in the Cheddar and Parmesan.
Add the sauce to the pasta and toss to combine. (The mixture will seem wet.) Dividing evenly, transfer the mixture to the prepared baking dishes and sprinkle with the crackers and parsley.
Bake until golden, 15 to 20 minutes. Serve with the green beans.
This dish can be frozen, unbaked and covered in plastic wrap, then foil, for up to 3 months. To bake, let thaw overnight in the refrigerator. Remove the plastic wrap, re-cover with foil, and bake at 350° F until heated through, 20 to 25 minutes for 6-ounce baking dishes or 50 to 55 minutes for an 8-inch square baking dish. Uncover and bake until golden, 5 to 10 minutes more.
Good for a crowd. Needs more baking time than the recipe specifies.