Put the hot dogs in a food processor and pulse until coarsely chopped. Heat the oil in a large nonstick skillet over medium-high heat, add a third of the onions and cook, stirring, until soft and slightly golden brown, about 4 minutes. Add the garlic and stir for 30 seconds. Add the chopped hot dog and cook, stirring occasionally, until lightly golden brown, about 5 minutes. Remove from the heat and let cool completely.
Preheat the broiler. Open the buns and arrange on a baking sheet. Broil until toasted on each side, 1 to 2 minutes per side. Cut into small pieces.
Whisk together the mayonnaise and poppy seeds in a small bowl. Toss the cucumber and celery salt together in a small bowl.
Layer the dip in a medium, 3-quart glass trifle bowl or your favourite small serving dish. Spread out the tomatoes on the bottom, then top, in even layers, with the peppers, toasted buns, a generous drizzle of mustard, the remaining onions, the hot dog mixture, the pickles, poppy seed sauce, relish and cucumber. Wrap and refrigerate until chilled, at least 1 hour or overnight. Garnish with another drizzle of mustard and serve with potato chips.