In a small bowl, combine the mustard, vinegar and oil, whisking until combined. Set aside.
Meanwhile, heat a large skillet over medium heat and cook hot dog slices, turning with a fork or tongs, until slightly browned on both sides.
In a salad bowl large enough to hold all of the ingredients, add the onion, lettuce, tomatoes and pickles and toss. Add dressing a little at a time, being careful not to over dress the salad. Season the salad with salt and pepper.
Place salad into four individual serving bowls, top with browned hot dog slices and serve.
