Step 1

Preheat oven to 400°F. Lightly coat an 8-by-8-inch baking dish with cooking spray.

Step 2

Heat 2 tablespoons oil in a large skillet over medium heat. Add onion and cook, stirring, until softened and starting to brown, about 3 minutes. Add broccoli and 1/4 teaspoon each salt and pepper and cook, stirring, until mostly softened, 3 to 4 minutes. Add garlic and cook, stirring, until fragrant, about 30 seconds. Transfer mixture to a bowl.

Step 3

Add 1 tablespoon oil to the pan. Sprinkle in flour and cook, stirring, for 1 minute. Gradually add milk and whisk constantly until the sauce thickens, about 2 minutes. Remove from heat and stir in lemon juice and the remaining 1/4 teaspoon each salt and pepper. Stir in Parmesan until melted. Add chicken, orzo and the broccoli mixture; stir until well coated; transfer to the prepared baking dish.

Step 4

Stir panko, lemon zest and the remaining 1 tablespoon oil in a small bowl. Sprinkle evenly over the casserole. Bake until browned and bubbling, 15 to 20 minutes.