• Preparation of Ingredients: Assemble all ingredients to ensure availability and ease of use during the cooking process.

Oven Preheating and Dish Preparation:

• Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius).
• Adjust the oven rack to the upper third position.
• Coat a 9x13-inch baking dish evenly with cooking spray.

Sautéing the Base:

• In a large nonstick skillet, melt butter over medium heat.
• Add the chopped onion, sliced garlic, Italian seasoning, crushed red pepper, and ¾ teaspoon of the kosher salt.
• Cook, stirring frequently, until the onions are softened and translucent, approximately 4 minutes.

Creating the Roux:

• Stir in the flour and continue to cook, stirring constantly, for 1 minute.
• Gradually add the milk, continuing to stir constantly to prevent any lumps, until the mixture slightly thickens, about 3 minutes.

Combining the Main Ingredients:

• Incorporate the cream cheese and the remaining 1 teaspoon of salt into the sauce, stirring until the cheese is fully melted and the mixture is homogeneous.
• Remove the skillet from the heat.
• Stir in the shredded chicken, cooked rice, drained spinach, lemon juice, and lemon zest until all ingredients are evenly distributed.

Baking:

• Transfer the prepared mixture into the greased baking dish, spreading it out into an even layer.
• Evenly sprinkle the shredded mozzarella and grated Parmesan cheeses over the top.
• Place the dish in the preheated oven and bake until the cheese on top is melted, approximately 15 minutes.

Broiling for a Golden Finish:

• After baking, turn the oven to broil.
• Broil the casserole for 2 to 3 minutes, or until the cheese is golden brown and bubbly.