Place potatoes in a medium saucepan, cover with cold water, and add salt. If using more than 1 variety, use a different saucepan for each to accommodate varying cooking times. Bring to a boil, and then reduce to a simmer and cook until fork tender, about 10 to 20 minutes.
While the potatoes cook, heat butter and oil in a medium skillet over high heat, and add onions. Reduce heat to medium, and saute until golden brown and slightly caramelized, 10 to 12 minutes.
Drain potatoes, and let cool briefly. With a paring knife, remove a decorative band of peel from the center section of each potato. Divide potatoes between 4 soup bowls and pour 3/4 cup buttermilk into each bowl. Top with caramelized onions, season with salt and pepper, and garnish with dill. Serve immediately.
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