Put potatoes in a large saucepan and cover with water. Add 1 tbsp. salt. Cover, bring to a boil, then reduce heat and simmer until tender when pierced with a knife, 7 to 8 minutes. Drain, transfer to a rimmed baking sheet, and let cool completely.
Cut onion into 1-in. chunks and separate into pieces. In a large bowl, combine oil with 1/2 tsp. each salt and pepper. Add potatoes, onion, and bacon and toss to coat.
On each of 6 flat metal skewers, about 14 in. long, thread 2 pieces of onion, a piece of bacon, and a potato, pressing them close together; repeat until you have 5 potatoes and end with bacon and onion.
For charcoal, ignite a full chimney of hardwood charcoal on firegrate of a charcoal grill. Spread in an even layer on firegrate and set cooking grate in place. For gas, heat a gas grill to medium-high (about 450°).
Grill skewers covered, turning often, until onion and bacon have some char to them, 10 to 12 minutes. Set skewers on a platter and sprinkle with parsley.