Preheat oven to 400°F. Toss bread cubes with 2 tablespoons olive oil and 1/4 teaspoon celery salt on a rimmed baking sheet. Spread in a single layer and bake until crisp and golden at the edges, 8-10 minutes, tossing once halfway through. Let cool completely.

Heat a large skillet over medium-high heat. Add sliced hot dogs and cook until well-browned and slightly crisp on the edges, about 4-5 minutes, stirring occasionally. Transfer to a plate and let cool slightly.

In a small bowl, whisk together yellow mustard, white vinegar, 3 tablespoons olive oil, and poppy seeds until emulsified. The dressing should be tangy and slightly thick.

In a large bowl, combine toasted bread, tomatoes, sweet relish, sport peppers, pickle pieces, and diced onion if using. Add the warm hot dogs and pour the dressing over everything. Toss gently but thoroughly, letting some of the bread absorb the juices while other pieces stay crisp.

Let stand at room temperature for 10-15 minutes so the bread can soak up the tomato juices and dressing. Just before serving, sprinkle with remaining celery salt and garnish with fresh dill. Serve immediately while textures are still varied.