Cook Pasta: Cook the pasta in a large salted pot of boiling water according to package instructions, until al-dente. Drain, rinse with cold water and set aside.: The pot should smell faintly of salt and boiling water, and the surface will dance with steady bubbles as you add the penne . When the pasta nears al dente you will notice a slight resistance when you bite into it, with a firm center that means it will finish perfectly once mixed with the warm sauce. Drain promptly and rinse with cold water to stop the cooking, especially if you are not tossing immediately. A common mistake is leaving pasta in the hot pot, which continues to cook it and can make the texture too soft, so transfer to a colander right away.
Make Sauce: In a large saucepan or Dutch oven, melt the butter over medium heat. Add the flour and Italian seasoning and whisk. Cook for 2 minutes while whisking continuously.: As you melt the butter in your saucepan or Dutch oven, you should hear a gentle sizzle and see a glossy pool form, signaling the perfect start for a roux. Stir in the all purpose flour and the Italian seasoning , whisking continuously until the mixture smells slightly toasty, about two minutes; this removes raw flour taste and blooms the herbs so their flavor becomes more integrated into the sauce. If you stop whisking, the flour can clump or brown unevenly, so keep moving the whisk to keep the texture even and smooth.
Finish Sauce: Add the milk and whisk until it thickens. Stir in the cheeses and whisk until well combined and smooth.: When you add the milk , pour in a slow, steady stream while whisking; the sound changes to a gentle bubbling and the mixture will begin to thicken as the starches swell. As the sauce becomes glossy and coats the back of a spoon, reduce heat slightly to maintain a gentle simmer and stir in the shredded cheese until it melts into a silky emulsion. The aroma will shift to rich dairy with bright herb notes, and the sauce should be velvety without graininess. If the sauce separates or becomes grainy, lower the heat and whisk vigorously, or add a small splash of warm milk to bring it back together.
Add Pasta: Add the penne to the pot and toss well making sure the pasta is fully covered in sauce.: When you fold the drained penne into the sauce, notice how each tube picks up a ribbon of cream, the surface glossy and smooth. Toss thoroughly so the sauce clings to every piece, and if the mixture seems thick, add a tablespoon or two of warm milk to loosen and ensure an even coating. Proper tossing makes the difference between pockets of dry pasta and a uniformly sauced dish; avoid dumping everything in and stirring roughly, which can break the pasta and create an inconsistent texture.
Garnish & Serve: Garnish with red pepper flakes and serve.: The final touch of red pepper flakes adds a sparky aroma and a faint heat that livens each bite, and the presentation should look creamy and gently puffed in the center. Serve immediately while the sauce is warm and glossy, because as it cools the texture firms and the experience changes. One frequent error is holding the dish too long before serving, which can make the sauce congeal and reduce the sensory pleasure of that first warm, melty mouthful.