Preheat the oven to 180 C / Gas 4. Lightly grease a baking dish.
Lay the chicken breast fillets out in the prepared baking dish in a single layer.
Bake the chicken in the preheated oven until it is no longer pink in the centre, about 20 minutes. Remove from the oven, allow to cool for a few minutes then cut into chunks and place back in the baking dish.
Meanwhile, bring a saucepan of water to the boil and add the broccoli. Cook until just soft, about 15 minutes. Drain and add to the chicken on the baking tray along with the mushrooms, salt, pepper and onion powder.
Mix the cheese sauce granules with the boiling milk and stir until thick and creamy and pour over the vegetables and chicken.
Bake in the oven for 10 to 15 minutes until the top begins to brown. Remove from the oven, serve up and enjoy.
Make sure to use fresh ingredients instead of frozen.