Mix all the sauce ingredients in a bowl until well combined. Set aside or refrigerate until needed.
Cook the Ground Beef:
Season the ground beef with salt and pepper. Form into thin patties roughly the size of your bread slices.
Cook the patties in a skillet over medium-high heat until fully cooked and slightly browned on each side. Place a slice of American cheese on each patty during the last minute of cooking to melt the cheese.
Make the French Toast:
Beat the eggs with milk, salt, and pepper in a shallow dish.
Soak each bread slice in the egg mixture, ensuring both sides are well coated.
Melt butter in a skillet over medium heat. Fry each bread slice until golden brown on both sides. Repeat with all bread slices.
Assemble the Big Mac French Toast:
Spread Big Mac sauce generously on one side of each French toast slice.
Build the sandwich by placing a cheesy beef patty on a sauce-covered French toast slice. Add shredded lettuce, minced onion, and sliced pickles. Top with another slice of French toast, sauce side down.
Serve immediately, cutting each sandwich in half if desired.