NOTE: USE MOSTACCIOLI PASTA OR LARGE ELBOW MACARONI (I USE SAFEWAY BRAND MOSTACCIOLI). DO NOT ADD ANY SALT TO THE BOILING WATER BEFORE ADDING PASTA (WHAT MAKES THIS RECIPE UNIQUE IS THE CONTRAST OF FLAVORS BETWEEN THE "SWEETER" UNSALTED PASTA AND THE MILD SALTINESS OF THE CHEESE SAUCE). USE ONLY HOMOGENIZED WHOLE MILK. USE 24 OUNCES SOLID MEDIUM TO SHARP CHEDDAR CHEESE (24 OUNCES = ONE 1 LB BRICK, PLUS ONE 8 OUNCE BRICK -- OR -- TWO 12 OUNCE BRICKS). USE ONLY POWDERED GRATED PARMESAN CHEESE. (STEP 1): PREHEAT oven to 350°F ; LIGHTLY butter or spray a 2.8 - 3 quart casserole dish; SHRED sharp white cheddar cheese on a medium grate; DIVIDE cheese into 3 mounds: One mound consisting of 1 cup cheese (for the garnish), and two larger mounds divided equally, using the remaining cheese; FINELY grate 1/2 cup Gruyere cheese.
INTO a medium microwave-safe bowl, add 4 1/4 cups whole milk; IMMERSE the 1 sprig fresh rosemary and 1 sprig fresh thyme into milk.
HEAT milk mixture in a microwave at 100% power, pausing at intervals to stir, until very hot but not boiling; WHEN hot, float the onion slice over milk and let steep for 2-3 minutes; REMOVE rosemary and thyme sprigs and onion slice and discard; STRAIN any debris if necessary; WHISK in 1 1/2 level teaspoons kosher salt, 1/4 teaspoon garlic powder, 1/4 teaspoon ground nutmeg, 1/4 teaspoon ground white pepper, and 1/8 teaspoon cayenne pepper.
COOK pasta in 5-6 quarts rapidly boiling salt-free water in a large pot, until al dente (tender but retaining firmness); DRAIN in colander then rinse with cold water (leave pasta slightly wet before using).
NOTE: WHEN LAYERING THE FOLLOWING INGREDIENTS, KEEP CASSEROLE LOOSELY ASSEMBLED WITHOUT PACKING, TO ALLOW THE SAUCE TO PERMEATE. (STEP 3): PLACE 1/2 of the pasta into casserole dish; SPRINKLE the first large mound of cheddar cheese over the pasta; ADD the remaining portion pasta over the cheddar layer; SPRINKLE the second large mound of cheddar cheese over the pasta again.
IN the previous emptied pot over a low heat setting, add the butter in tablespoons until melted; SPRINKLE in the flour and stir; RAISE heat to medium low and cook until light golden, bubbly and slightly nutty in aroma, stirring as needed (do not let mixture turn brown or you will need to wash the pot and start over!).
RAISE heat to medium-high; GRADUALLY whisk in the hot milk mixture; WHISK constantly until mixture begins to steam and bubble; REDUCE heat and cook 1-2 minutes until fully thickened; REMOVE from heat; STIR in the Gruyere cheese and 1/4 cup powdered parmesan cheese until blended and creamy.
POUR the sauce over layered casserole, using a flexible spatula to scrape all sauce from pot; SPREAD and smooth sauce evenly to cover entire casserole, neatly to the edges of baking dish.
TOP casserole with the remaining (1-cup) mound shredded cheese garnish , followed with 2 tablespoons parmesan cheese OR a buttered breadcrumb topping -- see recipe in description).
BAKE uncovered for 30 minutes; LOWER dish to bottom rack; TURN on broiler and keep an eye on it, until golden browned and crusty on top; COOL 10 minutes; SERVE and enjoy!