Place potatoes in a medium saucepan, cover with cold water, and add salt. (If using more than one variety, use a different saucepan for each to accommodate varying cooking times.) Bring to a boil, and reduce to a simmer; cook until fork-tender, 10 to 20 minutes.
While 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. For a decorative touch, use a paring knife to remove a band of peel from the center section of each potato. Divide potatoes between four 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.