For the dip: Cut the onion into slices, keeping the layers together in full rounds. Lightly oil a hot grill or cast-iron skillet and place the onions cut-side down on the heated surface. Cook on one side until completely blackened and charred. Turn and char the other side, about 5 minutes total.
When all the onion is cooked, cool it completely, and then roughly chop. Combine the onion, sour cream, mustard, hot sauce, Worcestershire and lemon juice in a food processor. Puree until thoroughly mixed but not smooth. (To make the dip without a food processor, finely chop the onions, and then blend with the remaining ingredients.) Stir in the chives and adjust the seasoning.
For the potato chips: Slice the potatoes thin (less than 1/8 inch) on a mandoline. Place the sliced potatoes in a bowl and rinse under cool running water until the water in the bowl is clear. Drain and dry very well with paper towels or spin in a salad spinner.
Heat a large pot of vegetable oil to 350 degrees F. Add the potatoes in batches and fry until crisped, about 3 minutes per batch. When they are brown, remove the potatoes from the oil and drain on paper towels. Season immediately with salt. When all the chips are cooked, toss in a bowl with the rosemary and freshly ground pepper.