Many supermarkets carry red, white, and blue fingerling potatoes these days, but if you can’t find them, everyday full-size red- or white-skinned potatoes will taste just as good.
Step 1: Cut the bacon, if using, into 1/4-inch slivers, put it in a small nonstick frying pan, and cook over medium heat until lightly browned and the fat is rendered, 3 to 4 minutes. Using a slotted spoon, transfer the cooked bacon to paper towels to drain.
Step 2: Make the salad dressing: Place the mayonnaise, mustard, vinegar, capers, scallions, and bacon, if using, in a large nonreactive serving bowl and whisk to mix. Set the salad dressing aside.
Step 3: Scrub the potatoes with a brush, but leave the skins on. Place the potatoes in an aluminum foil drip pan. Drizzle with the olive oil and season very generously with salt and pepper. Toss the potatoes to mix.
Step 4: Set up the grill for indirect grilling and preheat to medium-high. If using a gas grill, place all the wood chips or chunks in the smoker box or in a smoker pouch and preheat on high until you see smoke, then reduce the heat to medium-high.
Step 5: When ready to cook, if using a charcoal grill, toss all the wood chips or chunks on the coals. Place the pan with the potatoes in the center of the hot grate, away from the heat. Cover the grill and cook the potatoes until tender, about 40 minutes. To test for doneness, insert a metal skewer into a potato; it should pierce it easily. Remove the pan with the potatoes from the grill and let cool.
Step 6: Cut any large potatoes in quarters; cut medium-size potatoes in half. Leave small potatoes whole. Add the potatoes to the salad dressing and toss to mix. Taste for seasoning, adding salt, mustard, or vinegar as necessary; the salad should be highly seasoned. If the salad is too dry, add a little more mayonnaise. Sprinkle with the reserved scallion greens and serve at once.