Preheat the oven to 220C/425F/Gas 7.
Gently fry the onions in the oil in a saucepan and cook until soft. Stir in the butter and the flour. Mix thoroughly, cooking on a low heat for one minute.
Pour in the milk, gradually, until everything is combined. Cook for two minutes.
Add half the cheese to the sauce and reserve the other half.
Meanwhile, pan-fry the salmon steak for one minute on each side.
Lay the salmon steak in a small oven-proof dish.
Add the broccoli, carrots and cherry tomatoes to the dish.
Pour over the sauce and top with the rest of the cheese.
Bake in the oven for ten minutes or until bubbling and golden.