Preheat oven to 425ºF. Mix together all of the ingredients for the sauce. Store in the refrigerator until ready to serve.Arrange tater tots in a single layer on a rimmed baking sheet. Bake in preheated oven until starting to turn brown, about 20–25 minutes.Meanwhile, heat a large skillet over medium heat. Melt butter and add onion. Cook for 3 minutes. Add ground beef, season to taste with salt and pepper, and cook until no longer pink. When tater tots are nice and crispy, remove from the oven and sprinkle cooked ground beef on top in an even layer. Top with shredded cheese. Return to oven and bake until cheese melts, about 5–10 minutes.Remove from oven and top with lettuce, tomatoes, and pickles. Drizzle sauce on top. Serve immediately.