Preheat oven to 180°C.
Cook pasta according to instructions on packet, minus 3 minutes cooking time. Drain and set aside.
Combine cream cheese, lemon juice, zest and chives in a large bowl. Season to taste.
Stir hot pasta, spinach and flaked salmon through cream cheese mixture then transfer to a large ovenproof baking dish. Top with breadcrumbs and grated cheese.
Bake for 15 minutes until golden and serve with fresh watercress.