Preheat oven to 350. Lightly spray a 9×13-inch pan with cooking spray.: As the oven comes to temperature you should sense a steady warmth filling the kitchen, which sets the stage for even baking. Preheating ensures the casserole starts cooking uniformly, yielding a bubbly interior with a nicely browned top. A common mistake is skipping preheat, which leads to uneven baking and a soggy center. If your oven runs hot or cool, use an oven thermometer to confirm accuracy.
In a large skillet over medium heat, cook sausage until lightly browned, about 5 minutes. Remove from heat and set aside.: You will hear gentle sizzling and see the edges of the sausage take on caramelized color, releasing savory fat and aroma. Browning enhances flavor and adds textural contrast, and the rendered bits will mingle with the sauce later. Avoid crowding the pan, which steams rather than browns the slices; if that happens, brown in batches to maintain that crisp edge.
Whisk together soups, chicken broth, and cajun seasoning.: The mixture should become a cohesive, pourable sauce, with a smooth sheen and the scent of spices brightening it. Whisking eliminates lumps and allows the seasoning to distribute evenly, so every forkful tastes balanced. A mistake is under whisking, which leaves pockets of powder; take an extra moment to achieve a uniform texture.
Lightly spray a 9×13-inch pan with cooking spray. Layer 1/2 vermicelli, 1/2 chicken, 1/2 sausage, and 1/2 of soup mixture. Sprinkle half of the parmesan cheese on the pasta. Repeat layers and top with remaining parmesan cheese.: As you layer, you'll notice the strands of vermicelli pasta settling and the components nestling together. This layering helps the sauce penetrate the pasta and protein evenly. If you dump everything without layering, you risk clumps and uneven distribution; gentle, intentional layering prevents that.
Bake uncovered for 45 minutes, until bubbly.: The cheese will sit on top of the first layer and start to melt as the hot sauce makes contact. Its salty, nutty aroma should become noticeable, hinting at the golden top to come. Skipping this step or uneven sprinkling can lead to bare spots on the casserole top, so distribute the cheese uniformly.
Repeat layers and top with remaining parmesan cheese: Building a second layer ensures each bite has a mix of vermicelli pasta , chicken , and sausage, while the final cheese layer guarantees an appealing crust. Press gently so everything is compact but not mashed. A common error is overpacking the pan, which can prevent even bubbling; leave a bit of space for the sauce to expand.
Bake uncovered for 45 minutes, until bubbly: You want to see the sauce gently bubbling at the edges and the top turn a light golden brown, with the parmesan cheese slightly crisped. The bubbling indicates the interior has reached proper temperature and the flavors have married. Opening the oven too often prolongs cooking time and may cool the surface, so resist peeking until the last 10 minutes unless checking for excessive browning.