Preheat oven to 180 degrees celsius.
Steam broccoli florets until just tender, don't overcook as they will cook a little more in the oven.
Arrange broccoli in an oven proof dish, sprinkle over tuna evenly. Pour over the prepared cheese sauce and sprinkle with grated cheese.
Bake for 15 - 20 mins until golden on top and bubbling. Allow to cool for 5 mins before serving.