For the Bacon Vinaigrette: Render the bacon over medium heat until crisp. Drain, reserving ¼ cup bacon fat in the pan.
Add the onions to the fat and cook until softened.
Add the remaining ingredients and whisk together. Bring to a simmer and reserve.
For the Golden Potatoes: For each serving, combine 12 potatoes, ¼ cup oil, 4 cloves garlic and 1 sprig rosemary in a vacuum bag and seal.
Cook in a water bath with a circulator at 194°F for 1 hour.
Remove and use immediately or shock the bags in ice water and refrigerate.
For each serving: Cut one bag of hot golden potatoes into ½-inch pieces and plate. Add 4 ounces warm brisket, 2 tablespoons warm onions, 2 ounces vinaigrette, and 1 ounce horseradish sauce. Garnish with chives or herbs.