Preheat the Oven: Preheat your oven to 375°F (190°C).
Cook the Ground Beef:
In a large skillet, cook the ground beef and diced onion over medium heat until the beef is browned and the onion is translucent. Drain excess fat if necessary.
Season with salt, pepper, and Worcestershire sauce (if using).
Prepare the Sauce:
In a small bowl, mix together all the ingredients for the special sauce. Adjust seasoning to taste.
Assemble the Casserole:
In a baking dish, spread the cooked ground beef mixture evenly.
Sprinkle the chopped pickles over the beef.
Drizzle half of the special sauce over the top.
Sprinkle the shredded cheddar and mozzarella cheese evenly over the mixture.
Bake:
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
Garnish and Serve:
Remove from the oven and let it cool slightly. Drizzle the remaining special sauce over the top.
Sprinkle with sesame seeds (if using) for a classic Big Mac touch.
Enjoy:
Serve hot and enjoy the keto-friendly Big Mac flavors!