Grate the zucchini and place it in a colander or fine-mesh strainer set over a bowl. Sprinkle with salt and let sit for 10-15 minutes—this draws out excess moisture that would otherwise make the casserole watery. After the time has passed, gather the zucchini in a clean kitchen towel or cheesecloth and squeeze firmly, wringing out as much liquid as possible. This step is crucial for texture, so don't skip the squeezing! While the zucchini drains, grate the carrot and finely dice the onion, setting them aside.
In a large mixing bowl, combine the ground chicken with the squeezed zucchini, grated carrot, and diced onion from Step 1. Add the egg, which acts as a binder to hold everything together. Sprinkle in the garlic powder, smoked paprika, black pepper, salt, and Italian seasoning, mixing everything together gently but thoroughly until just combined—avoid overworking the mixture, which can make the casserole dense. I prefer to use my hands for this to ensure even distribution of seasonings, which gives a more consistent flavor throughout every bite.
Heat your oven to 400°F. While the oven preheats, lightly grease a 9x13 inch baking dish or similar-sized casserole dish. Transfer the chicken mixture from Step 2 into the prepared dish and use a spatula or the back of a spoon to level and pack it down gently into an even layer. Spread the sour cream evenly over the top of the mixture—this will create a creamy layer that helps keep the casserole moist during baking.
Place the casserole in the preheated 400°F oven and bake for 35 minutes, until the chicken is cooked through and the top is lightly browned. Remove from the oven and evenly distribute the shredded cheddar and grated Parmesan cheese over the top—I like to use cheddar shredded from a block rather than pre-shredded, as it melts more smoothly without the anti-caking agents. Return to the oven and bake for another 10 minutes until the cheese is melted and bubbly.
Remove the casserole from the oven and let it rest for 5 minutes before serving—this allows the residual heat to set the structure. Garnish generously with fresh chopped parsley and sliced scallions for brightness and color. If using bacon bits, sprinkle them over the top for added smoky crunch and a professional finish. Serve warm directly from the dish or portion into individual bowls.