Preheat the oven to 400 degrees F (200 degrees C). Lightly grease a 24-cup mini muffin pan with cooking spray.

Heat a large, nonstick skillet over medium-high heat. Add ground sirloin and cook, crumbling with a wooden spoon and stirring often until evenly browned, about 5 minutes.

Sprinkle with 1/2 teaspoon salt, pepper, garlic powder, and Worcestershire sauce. Cook for 1 more minute, then remove from heat and set aside.

In a bowl, stir together mayonnaise, mustard, relish, onion powder, paprika, vinegar, sugar, and the remaining salt and pepper until combined. Refrigerate the dipping sauce until ready to serve.

Place about 1 tablespoon of cheese in the bottom of each muffin cup, working it against the sides if possible.

Divide the meat mixture evenly among the muffin cups and top with minced onion.

Add a slice of pickle to each cup and top with the remaining cheese. Sprinkle with sesame seeds.

Bake in the preheated oven until the cheese begins to brown around the edges, about 15 to 20 minutes.

Allow to set up in the pan for 3 to 5 minutes before removing to a paper towel-lined plate. Top with more sesame seeds if desired and serve with the Big Mac dipping sauce.