In a heavy saucepan, melt the butter over medium heat.
Add the onion and sauté until tender and translucent, 8-10 minutes.
Add the potatoes and stock and bring to a boil.
Reduce the heat to low and simmer, uncovered, until the potatoes are tender, 15-20 minutes.
Remove from the heat and add the potatoes, onion, and a little of the stock to a blender. Puree until smooth.
Return the puree to the pan holding the remaining liquid and reheat gently.
The soup will be thick. Thin it with the milk, season with the salt and pepper, then pour into a warmed soup bowl. Add sun-dried tomatoes. Garnish with mint and a drizzle of oil from the sun-dried tomato jar, if desired.