Heat oven to 400° F.
On a rimmed baking sheet, toss the potatoes, oil, garlic, ½ teaspoon salt, and ¼ teaspoon pepper. Arrange in a single layer and roast, tossing once, until golden brown and crisp, about 45 minutes.
When the potatoes have 15 minutes left to cook, place the salmon on a second baking sheet, season with ½ teaspoon salt and ½ teaspoon pepper, and roast until opaque and beginning to flake, 10 to 12 minutes total, depending on the thickness. Divide the potatoes among individual plates and serve with the salmon, dill, and lemon.