Preheat oven to 400 degrees F. Spray a baking dish with nonstick spray.: The oven should be fully hot when the assembled casserole goes in so the top crisps properly and the interior becomes bubbly; you will notice the oven hum and a dry warmth when it is ready. Ensuring the baking dish is well sprayed prevents sticking and makes serving clean, crisp slices easier. A common mistake is skipping the preheat and putting the dish into a not fully heated oven, which can lead to a soggy topping rather than a golden crust.
Bring water to a boil and prepare pasta according to directions, shaving 1-2 minutes off of the cooking time.: You want the pasta to be slightly underdone so it finishes cooking in the oven while absorbing the sauce; the water should roar with bubbling before adding the noodles, and salted water will season the pasta core. Drain when the pasta is just shy of tender, because overcooking here results in mushy texture after baking.
While pasta is boiling, heat butter in a medium saucepan over medium heat. Once sizzling, add flour and whisk constantly to create a roux, until golden and bubbly, about 2 minutes. Pour in milk and whisk constantly, stirring until the mixture slightly thickens. Reduce heat to low and add in 6 ounces of the fontina, all of the gorgonzola and about 6 ounces of the other blue. Stir until cheese melts and sauce is thick. Taste and season with a bit of salt and pepper if desired.: As the butter hits the pan you will hear a gentle sizzle and smell a toasty aroma when the flour cooks; whisking constantly prevents lumps and creates a satin smooth base. Adding the milk gradually yields a glossy texture, and when the cheeses begin melting you will see the sauce turn silky and slightly glossy, with ribbons when you lift the whisk. This technique matters because it ensures an even, creamy sauce that clings to the noodles. A typical pitfall is overheating once the cheese is added, which can cause the sauce to separate, so keep the heat low and stir gently.
Add noodles to the baking dish. Pour cheese sauce over top, tossing to coat all the noodles. Sprinkle remaining fontina and blue over top evenly, then cover in breadcrumbs. Bake for 30-35 minutes, or until top is golden and crunchy.: When you combine the noodles and sauce, use gentle folding motions so each piece is coated; you should see ribbons of creamy sauce enveloping the pasta and hear a quiet slosh as it moves. Even distribution of the remaining fontina and blue on top ensures that pockets of melty character appear across the surface, and the breadcrumb layer will form a golden protective crust. Avoid packing the casserole too densely, because that can prevent even browning and texture contrast.
Bake for 30-35 minutes, or until top is golden and crunchy: During baking the kitchen fills with a warm, savory perfume and you will notice the edges gently bubbling and the breadcrumbs browning to a deep gold; the sound is subtle, a soft crackle as the topping crisp up. The oven's heat finishes cooking the pasta and melds the cheeses into a cohesive, indulgent filling. A common misstep is opening the oven repeatedly, which lowers the temperature and can delay the crisping process, so trust the time and peek near the end instead of frequently checking.