Preheat oven to 180°C, fan forced.
Cook pasta following packet directions. Drain. Return back to pan.
Melt butter in a medium saucepan over medium heat.
Sprinkle in flour, cook, stirring, for 1 minute, remove from heat.
Slowly pour in milk, stirring to combine, return to heat.
Cook, continue stirring, until sauce boils and thickens.
Add egg, simmer for 3 minutes.
Add salmon, onions, mustard, lemon juice and salt and pepper to season. Stir.
Simmer for a further minute.
Pour salmon mixture into saucepan of pasta, mix to combine.
Spoon mixture into casserole dish.
In a small bowl or jug combine breadcrumbs, cheese and reserved spring onion tips.
Sprinkle breadcrumb mixture over top of the salmon mix.
Bake in oven for 25-30 minutes or until golden.
Serve sprinkled with spring onions.
