In a large saucepan or Dutch oven, combine potatoes, salt and enough cold water to cover by 1 in. Bring to a boil; reduce heat and simmer until potatoes are slightly cooked but still al dente, 3-4 minutes. Drain. , Meanwhile, in a large skillet, melt 3 tablespoons butter over medium heat. Add onion; cook, stirring frequently, until golden brown, 10-12 minutes. Remove onion; set aside. In same skillet, melt remaining 3 tablespoons butter. Add potatoes; cook, stirring occasionally, until potatoes are golden brown, 8-10 minutes. Add onions, rosemary and pepper; toss to combine. Cook until heated through.