How to Make It
Place potatoes in a large pot and add cool water to cover by 1 inch. Add 2 tablespoons salt; bring to a boil over high. Cook until just fork-tender, about 15 minutes. Drain and let cool to room temperature. Cut potatoes in half lengthwise; place in a large bowl with onions.
While potatoes cool, cook bacon in a large skillet over medium-high, stirring often, until crispy, about 12 minutes. Remove bacon with a slotted spoon, and drain on a plate lined with paper towels. Reserve 5 tablespoons bacon drippings.
Toss potatoes and onions with 3 tablespoons reserved bacon drippings. Sprinkle with pepper and 1 1/2 teaspoons salt; toss to coat.
Preheat grill to very high (500°F and up). Place potatoes and onions in a grill basket, and grill, uncovered, until lightly charred, about 3 minutes per side. Remove from heat. Pull onion halves into individual petals; place in a large bowl. Add grilled potatoes, cooked bacon, parsley, and chives, and gently toss. Stir together lemon juice, mustard, 2 tablespoons reserved bacon drippings, remaining 1 1/2 teaspoons salt, and hot sauce, if using. Pour over potato mixture, and fold gently to combine. Transfer to a serving platter. Top with dollops of sour cream and dill sprigs.