Preheat oven to 350˚F.: As you let the oven come to temperature you will notice a warm, dry scent in the kitchen that signals the air is ready to produce even browning. Preheating is essential because it ensures the casserole begins cooking immediately and the top can develop that inviting golden color. If you skip preheating, the dish may bake unevenly and the center can remain too soft while edges overbake. A common mistake is placing the dish into a cold oven, which lengthens overall cook time and alters texture. Make sure your oven rack is set to a middle position so heat circulates around the dish.
In a large bowl, combine the cream of mushroom soup, onion, mayonnaise, and eggs. Stir in the onion, 1 cup of the cheese, and the chicken until mixed.: As you stir, the mixture should look glossy and slightly thick, with the diced onion softened into the sauce and the eggs starting to bind everything together. The aroma will be savory, with mushroom notes rising from the cream of mushroom soup . Combining ingredients in a single bowl ensures even distribution, so each spoonful will have chicken , cheese , and sauce. If you overmix, the texture can become overly compact, so stir just until combined. Watch for lumps of mayo or unincorporated egg whites, and gently fold them in to maintain a smooth sauce. A frequent error is not breaking up the chicken enough, which leads to uneven bites; shred it finely so it disperses throughout the mixture.
In a 2 quart casserole dish (or 9×9 inch) spread the broccoli over the bottom of the dish. Pour the soup mixture over the broccoli. Then top with the remaining cheese.: Laying the broccoli first creates a bed that cooks into tender florets but keeps the top saucy rather than soggy. When you spoon the mixture, you will hear a gentle plop as the sauce settles over the greens, and visually the white sauce will contrast with the vivid green. Scatter the remaining cheese evenly to create a golden crust as it melts. If you pile the filling too thick in one spot, that section may remain undercooked, so spread everything evenly. Avoid using a dish that is too deep unless you adjust baking time, because a deeper pan will require longer cooking to set the center without burning the edges.
Bake 45-55 minutes or until the top is golden brown.: During baking the kitchen will fill with a toasty, cheesy aroma and you will likely hear gentle bubbling from the center as steam escapes. Look for the top to turn a warm golden brown and for bubbling around the edges, which indicate the interior has reached a hot, set temperature. The longer end of the time range will yield a slightly firmer filling and a more pronounced crust, while the shorter end keeps it creamier. A common mistake is removing the casserole too soon, leaving a liquid center; probe the middle with a spoon to check for gentle jiggle rather than runniness. If the top browns too quickly, tent loosely with foil to allow the center to continue cooking without burning the cheese. Let the casserole rest for a few minutes after baking so it firms up and slices cleanly.