Preheat oven to 400°F. Place potatoes in a large pot, cover with water and add 1 Tbsp. salt. Bring to a boil and cook until potatoes are tender, about 10 to 15 minutes.
While potatoes are cooking, line a baking sheet with heavy-duty aluminum foil. Place salmon fillets on foil, skin side down. Brush with oil and sprinkle with salt and pepper. Roast until just opaque in center, 14 to 16 minutes. Remove from oven and loosely tent with foil to keep warm.
Drain potatoes and return to pot. Mash over low heat until smooth. Stir in sour cream, milk and dill. Season with salt and pepper to taste. Serve immediately with salmon.