Ensure the broccoli is fully thawed before beginning. To quickly thaw broccoli, microwave it at 50% power until no longer frozen but cool to the touch.: The thawed broccoli should feel cool and pliable, not icy. When you press a floret gently it should release any trapped ice crystals, and the texture will be slightly limp rather than rock hard. This matters because frozen pockets will turn to water during baking, thinning the sauce and making the casserole watery. A common mistake is rushing this step, which leads to excess liquid pooling in the pan; avoid that by setting the broccoli in a colander and pressing out moisture if needed.
Preheat oven to 350°F. Coat a 9×13-inch baking pan with nonstick cooking spray. Set aside.: You should feel the warm oven air when you open the door, and the pan will be dry and ready to receive the mixture. Preheating ensures even cooking and helps the topping brown reliably. If you skip preheating the casserole can bake unevenly and lengthen the required time. A pitfall to watch for is using a pan that is the wrong size, which can make the casserole too thin or too deep, altering cooking time.
In a large bowl, whisk the egg, cream of chicken soup, mayonnaise, garlic powder, onion powder, pepper, and paprika together.: As you whisk you should notice the sauce becoming uniformly smooth and slightly glossy, with the spices evenly distributed. This emulsion coats the broccoli better and prevents clumps of seasoning. The why here is simple, even coating yields consistent flavor in every bite. A common error is under-whisking, which leaves pockets of seasoning or uneven texture; whisk until smooth and well combined.
Add in the broccoli and cheddar cheese, and stir gently until the broccoli is well coated in the sauce.: When you fold in the broccoli and cheddar you'll see the florets take on a creamy sheen and the cheese begin to nestle into the crevices. Gently pressing the mixture helps the sauce cling to each piece. The tactile cue is that the broccoli should look glossy and the cheese should be distributed but not fully melted. Stirring too vigorously can break the florets apart, producing a mushy texture, so be gentle to preserve integrity.
Transfer to your prepared pan and smooth into an even layer.: Use a spatula to spread the mixture until the surface is level and compacted slightly, which helps the casserole bake evenly. The finished surface should be uniform, with no large air pockets, so the topping sits flush. If you leave a lumpy surface the topping may brown unevenly. One mistake is overpacking the pan which can slow heat penetration and increase bake time, so aim for an even, but not slammed, layer.
Top with the crushed crackers and drizzle with butter.: Scatter the crushed Ritz crackers evenly so they form a continuous layer, then drizzle the melted butter to moisten the crumbs. Watch for an even sheen across the top, because the butter encourages golden browning and prevents dry spots. This step creates the signature contrast of crunchy topping to creamy interior. A common slip is uneven distribution of butter, which leaves parts of the crust pale and soft, so be sure to drizzle deliberately and spread crumbs as needed.
Bake for 35-45 minutes, until the crackers are golden and the casserole is warmed through.: During baking you'll notice the kitchen fill with a warm, toasty aroma and the topping will change color from pale to a rich golden brown. The edges may bubble slightly where the sauce meets the pan, and the interior will feel set when a knife inserted near the center comes out mostly clean. This final heat melds flavors and crisps the topping. If the crust browns too quickly before the interior is hot, tent the pan with foil to prevent burning while the center finishes cooking. A typical oversight is pulling the casserole out too early, resulting in a lukewarm center, so test the middle for warmth before serving.