1.Preheat oven to 200°C (180°C fan-forced).
2.Cook pasta in large saucepan of boiling water until tender, drain.
3.Meanwhile, melt butter in medium saucepan, add flour, cook, stirring, until mixture thickens and bubbles. Gradually add milk, stirring over medium heat until sauce boils and thickens. Stir in peas, ¼ cup parmesan and ¾ cup cheddar cheese.
4.Combine sauce mixture with pasta and salmon in shallow 2.5-litre (10-cup) oiled ovenproof dish, sprinkle with remaining combined cheeses. Bake, uncovered, in oven, about 20 minutes or until browned lightly.