Place potatoes in a steamer. Fill a large pot with 1 to 2 inches water and place steamer in pot. Bring water to a simmer, cover, and steam potatoes until tender and easily pierced with a knife, 20 to 25 minutes.
Meanwhile, melt butter in a large skillet over medium heat. Add onion and coarse salt and cook, stirring occasionally, until onion is softened and golden, about 20 minutes.
Divide potatoes and onions evenly among 4 bowls. Pour 1/3 cup buttermilk over each serving, season with flaky salt and pepper, and garnish with dill.