Bring a large pot of salted water to a boil. Add potatoes and cook until tender but still firm, about 15 minutes. Drain, cool and chop.
In a medium saucepan, cook onions, garlic and butter over medium heat until vegetables become transparent in color.
Add onion/garlic mixture, salmon and thyme to cooked potatoes. Mash all together with milk, adding just enough to bring about a mashed potato consistency. Spread evenly into pie shell.
Place second pie shell over top of salmon mixture. Cut slits into the upper shell, then pinch edges of both pie shells together. Bake on lower rack of oven for 45 minutes.
