Place frozen burgers in a cast-iron skillet and top with thinly sliced onions. Sear burgers for about 5 minutes. until browned and cooked throughout.
Butter the Texas toast, then, in this order, add 1 slice of cheese, the smash burger, sliced pickles, and a second slice of cheese.
Add butter to another skillet and bring to medium heat; cook the grilled cheese, flipping halfway through, until the cheese has fully melted and it’s golden on both sides.
Serve with your favorite sauce for dipping!