Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius).: You'll notice a warm, steady hum as the oven climbs to 375 degrees Fahrenheit, which ensures the casserole bakes evenly and the top gets nicely golden. The consistent oven heat transforms the sauce and melts the reserved cheese into a crisp, bubbling finish. A common misstep is putting the dish into a cold oven, which can lead to uneven browning or extended bake times.
Boil the pasta until al dente, according to package directions. Drain. If it’s going to be awhile before you make the sauce, toss in a little butter or olive oil to help keep the pasta from sticking together.: Watch the pasta as it boils; it should be tender but still have a slight bite when you test it, which helps it hold shape in the casserole. Drain it well so the sauce does not become diluted, and toss with a little butter to prevent sticking. The telltale sign you boiled it right is a firm center that gives slightly when bitten. Overcooking will leave you with a mushy final texture, so avoid boiling beyond the recommended time.
In a large saucepan over medium heat, melt the butter. Add the flour, dry mustard, salt, and black pepper. Stir constantly over medium heat for about three minutes.: As the butter melts, it should foam and then calm, releasing a nutty aroma. This is the moment to add the flour , salt, dry mustard, and black pepper so the roux can develop. If the butter browns too quickly, lower the heat to prevent a burnt flavor, which would spoil the sauce.
Switch to a whisk and continue stirring while slowly pouring in the milk. Whisk frequently for about 5 minutes, and then constantly for another 5 minutes or so, until the sauce heats and thickens.: Stir these together until they are fully incorporated, cooking for about three minutes to remove the raw flour taste and to build a base that will thicken the milk . You want a smooth paste that smells toasty rather than floury. If lumps form, whisk vigorously to smooth them, as lumps can create grainy sauce.
Remove from heat and stir in 1 cup of the smoked cheddar, 1 cup of the medium cheddar, jalapeno peppers, and Tabasco sauce, stirring until the cheese has melted.: As you stream in the milk , whisk constantly so the sauce emulsifies and thickens evenly. The mixture will move from thin to velvety and should coat the back of a spoon. If the sauce stays thin, keep simmering gently, but do not boil rigorously, as high heat can separate the dairy.
Add the pasta and toss until completely coated with the jalapeño cheese sauce.: You will hear a quiet simmer and see the sauce gain body, slowing its drip off the spoon. This is when starches swell and the texture becomes creamy. Rushing this step or turning the heat too high can cause scorching; scrape the bottom regularly to prevent burning.
Add half the pasta to a two quart casserole dish and sprinkle on half of the remaining cheeses.: The moment you add the shredded cheddar , you should see it melt into luscious ribbons, releasing a cheesy scent. Folding in the diced jalapeno disperses bursts of heat; the Tabasco brightens the whole mixture. If the cheese clumps or the sauce looks separated, return to very low heat and stir gently until smooth.
Add the remaining pasta and sprinkle on the remaining cheese.: The best visual cue here is glossy pasta where every curve is slick with sauce. Toss gently so the pasta does not break, ensuring every bite will be richly flavored. If some pieces look dry, add a splash of milk to loosen the sauce before baking.
Place in the oven and bake uncovered for 25-30 minutes, until bubbly and golden brown.: Layering encourages even melting and a pleasing contrast of textures. The cheese on top of the first layer begins to integrate as the dish rests. A common oversight is packing the dish too tightly, which can prevent hot air from circulating and lead to uneven baking.
Add the remaining pasta and sprinkle on the remaining cheese: The final cheese layer is what will brown and become crisp in the oven, so distribute it evenly for a uniform golden finish. Look for a smooth, slightly domed surface before baking. If the cheese is uneven, parts will brown faster than others.
Place in the oven and bake uncovered for 25 to 30 minutes, until bubbly and golden brown: As it bakes, the sauce will bubble around the edges and the top will develop golden brown spots that hint at caramelized cheese. You should hear gentle bubbling and see steam escaping when you open the oven briefly. Avoid opening the oven too often, which can lower the temperature and extend baking time; instead, check visually near the end of the time window.