1. Combine in a very large bowl: potatoes, eggs, bacon, onions, string beans, tomatoes, and eggs and toss.
2. In a small bowl, whisk vinegar and mustard together, then slowly whisk in oil, drizzling in slowly.
3. Pour dressing over potato salad in the large bowl.
4. Season with 2 teaspoons of salt and 1 teaspoon of ground pepper.