In a large mixing bowl, add the ingredients for the vinaigrette. Whisk until they come together. Reserve. In a large pot, boil the potatoes until they are fork tender, about 10-15 minutes. Drain. Drizzle 2 Tbsps of the reserved dressing over the hot potatoes. This will allow the potatoes to absorb the flavor. Reserve.
In a separate bowl, toss the red onion and tomatoes with 1 Tbsp of the reserved vinaigrette. Drizzle the hard cooked eggs with about 1/2 tsp of the vinaigrette. Toss the romaine with the remaining vinaigrette. To assemble: place the romaine on the bottom of the platter. Arrange the ingredients, separately on the romaine. Top with the sardines. Serves 4.