Cook beef: Add the beef to a large pan and brown over medium-high heat for 3-4 minutes, smashing the beef to crumble. Add the onion, salt & pepper and cook for another 4-5 minutes or until the onion is softened and the beef is browned, cooked through and slightly crispy.
Make a big mac sauce: whisk all the ingredients in a medium bowl.
Assemble: Divide the lettuce into 4 bowls, top each bowl with ¼ the browned beef, onion, cheese, tomato and pickled. Drizzle with big mac sauce and serve immediately.
Substitutions: You can swap the ground beef for ground chicken, ground turkey or any other ground beef that you like. Topping Ideas: Add your favorite toppings like jalapenos, avocados, sour cream, or roasted veggies.