Step 1Heat the oven to 375 degrees. In a large bowl, toss the potatoes with the garlic, olive oil, 1 teaspoon salt and several grinds of pepper. Place the potatoes in a single layer on a parchment-lined baking sheet. Roast the potatoes until golden brown and tender, about 1 hour, tossing occasionally. Remove and cool.
Step 2While the potatoes are roasting, cut the bacon crosswise into one-fourth-inch pieces. Cook the bacon in a large saute pan over medium heat until the fat is rendered and the bacon is crisp, about 15 minutes. Stir frequently, watching that the bacon does not stick to the bottom of the pan. Remove from heat and drain the bacon on a paper towel-lined plate, reserving the grease for another use.
Step 3In a medium bowl, whisk together the mayonnaise, mustard and red wine vinegar. Season with one-half teaspoon salt and several grinds of black pepper. Taste and adjust mustard, vinegar and seasoning if desired.
Step 4In a large bowl, combine the potatoes and bacon with the sliced onion. Gently stir in enough dressing to coat, then stir in the capers. Taste and adjust seasoning if desired.
Step 5Cover and refrigerate the salad for at least 2 hours to allow the flavors to develop. The salad will keep for 3 days, refrigerated. Allow the salad to warm slightly at room temperature before serving.