Recipe adapted from Danny Meyer's The Union Square Cafe Cookbook (with Michael Romano; HarperCollins; 1994).
Preheat oven to 375 degrees. In a medium pot, cover potatoes with 6 cups water. Add garlic, rosemary, parsley, and thyme sprigs, peppercorns, 1 tablespoon salt, and pinch of red-pepper flakes. Bring to a boil over high. Reduce heat and simmer until potatoes are tender but not mushy, 10 to 12 minutes. Remove from heat and let potatoes cool in cooking liquid, about 3 hours.
Meanwhile, place bacon on a baking sheet and cook until crisp, about 20 minutes. When cool enough to handle, cut crosswise into 1/4-inch pieces.
Reserve 1/4 cup potato cooking liquid; drain potatoes and discard herb stems. In a large bowl, combine potatoes, bacon, and scallions.
In a small bowl, whisk together reserved cooking liquid, oil, vinegar, chopped parsley, 1 1/4 teaspoons salt, and 1/4 teaspoon pepper. Pour dressing over potatoes and toss to combine. Serve at room temperature (or refrigerate, up to overnight).