Preheat your oven to 375°F (190°C) and position the rack in the center.

Heat olive oil in a large skillet over medium heat. Add the diced onion and cook for 3 to 4 minutes until translucent and soft, with edges just starting to brown. Add minced garlic during the last minute, stirring constantly to prevent burning.

In a large mixing bowl, combine shredded chicken, sliced zucchini, sautéed onion and garlic mixture, mayonnaise, sour cream, Italian seasoning, salt, pepper, and paprika. Mix well until all ingredients are evenly coated.

Grease a 9x13-inch baking dish with cooking spray or olive oil. Spread the mixture into the dish and press down gently to eliminate air pockets and create a level surface.

Sprinkle mozzarella and Parmesan cheeses evenly over the top, covering the entire surface including the edges for extra crispy bits.

Bake for 25 to 30 minutes until the cheese is golden and bubbly with brown spots, and a fork slides easily through the zucchini.

Remove from oven and let rest for 5 to 10 minutes. Garnish with fresh chopped parsley right before serving.