Bring a medium saucepan of water to the boil over high heat. Add broccoli and cook for 1 minute until vibrant. Drain, refresh broccoli in cold water and set aside to dry.

Preheat oven to 180°C/160°C fan-forced. Arrange broccoli, chicken, mushrooms and green onions in a large (about 8 to 10 cup) greased baking pan. Set aside.

Dry the saucepan used for the broccoli. Add butter to pan and heat over medium-low heat until bubbling. Add garlic and cook for 30 seconds. Add flour and using a small whisk, stir constantly for 2 minutes until frothy. Decrease heat to low, and gradually whisk in milk and stock. Cook, stirring, for 4 to 5 minutes until sauce boils and thickens. Stir in 1 cup cheese. Season to taste.

Pour cheese sauce over broccoli mixture. Scatter over breadcrumbs and remaining ½ cup cheese. Bake for 35 to 40 minutes or until chicken is just cooked through and sauce is bubbling and serve.