Boil potatoes in salted water, until tender, approximately 15-20 minutes. Drain.
Meanwhile, melt butter in a pan over low heat; add onions and cook until softened, about 3 to 5 minutes.
Mash drained potatoes with milk in a bowl; add salmon, cooked onion, salt, dried savory (or thyme) and pepper. Mix well. Let cool.
Preheat oven to 200 C / Gas 6.
Arrange one portion of pastry in a pie dish. Spoon the potato-salmon mixture on top, then cover with the second portion of pastry. Trim and pinch edges to seal the pie, then use a knife to make 2 or 3 slits at the top of the pie for steam to escape while cooking.
Mix egg with a little bit of milk; brush the top of the pie. Bake for 25 minutes, or until the pie is cooked through and golden brown.