Squeeze all water possible out of the spinach.: You will feel the weight of the thawed spinach change as you wring it, and the pile will become noticeably drier and denser. This matters because excess moisture will steam the casserole and prevent a creamy texture, instead making it soggy. A tip I use is wrapping the spinach in a clean kitchen towel and twisting firmly until very little water remains, the towel will feel warm and damp. If you skip this the filling may separate while baking, causing a watery gap at the edges. Common mistake, especially when rushed, is squeezing only a little and assuming it is fine, which leads to a runny bake.
Mix chicken, spinach, cream of mushroom soup, sour cream, milk, parmesan cheese, garlic, and chicken bullion together until combined.: As you stir the mixture the aroma of pressed garlic and the savory note from the soup will bloom, and the texture should become uniformly creamy with flecks of green and striations of shredded chicken . This step builds the flavor base, and thorough mixing ensures each bite carries the same balance. Use a rubber spatula and fold gently until you no longer see pockets of dry ingredients, the surface should look glossy and cohesive. If the mixture seems overly thick add a touch more milk to reach a spoonable consistency. A frequent error is under mixing, leaving clumps of sour cream or cold pockets, which results in uneven texture after baking.
Spread evenly in a medium casserole dish.: When you transfer the filling, press it into a level layer so the heat distributes evenly during baking, and you will see small air pockets flatten out into a smooth plane. An even surface encourages uniform browning and prevents edges from drying faster than the center. Use the back of a spoon to smooth the top, and the dish should feel balanced when you move it. If you pile the center higher than the edges the middle may stay softer and need extra time. A common mistake is not centering the dish in the oven, which causes uneven browning and melting.
Bake covered at 375˚F for 30 minutes. Uncover, then add mozzarella cheese and bake an additional 5-10 minutes or until the cheese melts.: As the casserole heats you will notice a gentle bubbling and the scent of roasted garlic deepening, and the sides will quietly thicken as the sauce sets. Covering traps steam, allowing the interior to cook through without over browning the top, which keeps the filling moist and cohesive. The temperature should be steady, and the center will begin to feel set when you gently jiggle the dish. Avoid opening the oven repeatedly, or heat will escape and extend cooking time. A typical pitfall here is baking uncovered from the start, which can dry out the surface before the center cooks.
Garnish with parsley if desired and serve warm. Enjoy!: Once uncovered you will hear a soft, excited bubbling as moisture escapes and the top readies itself for browning, then after sprinkling the mozzarella it will melt into an inviting glossy sheet. Watch for the cheese to become uniformly molten and to develop faint golden spots, which signal the perfect finish. This final brief blast of heat brings textural contrast, the gooey melt against the creamy base. Overbaking at this stage will brown the cheese too much and dry the surface, so plan to check at the five minute mark and adjust as needed.
Garnish with parsley if desired and serve warm. Enjoy!: The garnish will add a fresh scent and a pop of color that brightens each serving, and when you spoon out the casserole you should see ribboned chicken and verdant flecks of spinach surrounded by a silky sauce. Serving warm helps the texture hold, and the contrast between the bubbling interior and slightly cooled top is lovely. One common mistake is letting it sit too long before serving, which can cause the texture to firm up and lose its lusciousness.
