Preheat oven to 350º F. Spray an 8-inch square casserole dish with cooking spray. Cook broccoli in microwave according to directions; set aside.
Heat olive oil over medium-high heat, add chicken and cook for about 10 mins in oven-ready deep pan.
In the meantime, steam cut up broccoli in pan with small amount of water.
Add onion and mushrooms to chicken and saute for about 5 mins. Add steamed broccoli back in.
Add flour, stir to combine and cook for 1 min. Add milk and cook for 2 mins.
Sprinkle flour over chicken and mushroom mixture and stir constantly, cooking for one minute. Stir in milk. Bring to a boil and cook for 3 minutes, or until thick and bubbly.
Add yogurt, salt and pepper; stirring lightly to combine.
Sprinkle top with cheese and bake for 20 minutes, or until cheese melts and bubbles around the edges.
Serve and enjoy!