Melt butter in a saucepan over medium heat until foaming. Add flour. Cook, stirring with a wooden spoon, for 1 minute or until bubbling.
Add milk, 1/4 cup at a time, stirring constantly. until mixture bubbles and thickens.
Remove from heat. Stir in 1/2 cup cheese.
Preheat oven to 180°C - 160°C fan-forced.
Cut the tomatoes into cubes and slice salmon into chucks. Combine onions, salmon and tomatoes in a bowl. Season with salt and pepper. Spread half the rice over the base of a greased 7cm-deep, 24cm (base) square baking dish. Arrange half the salmon mixture over rice. Spoon over the sauce and sprinkle cheese all over.
Bake for 25 to 30 minutes or until golden. Stand for 5 minutes. Serve with parsley.