The Theta burger is an Oklahoma smash burger defined by a single mandatory topping that sounds like it should not work: a hickory-smoked barbecue sauce and pickle combination on a thin griddled patty. It is a codified order rather than a freeform build, which is what separates it from a generic cheeseburger with sauce. The smash patty is the base technique, a loose ball of beef pressed hard onto a hot flat-top so it develops a deep, lacy seared crust and cooks thin and fast, but the Theta designation specifically means that crust is dressed with a smoky barbecue sauce and pickle rather than the usual ketchup-and-mustard frame, and that fixed pairing is the sandwich's whole identity.
The craft is in the smash and the timing of the sauce. The beef is pressed onto the griddle with enough force and heat to maximize the Maillard crust, because a thin patty has little interior and the crust is where nearly all of its flavor lives. Cheese is laid on while the patty is still on the steel so it slackens into the crust and seals the surface rather than sitting on top, which also slows the juice from bleeding into the bun. The barbecue sauce is the structural counterpart to that crust: its smoke and tang stand up to a hard-seared patty in a way a milder condiment would be lost against, and the pickle supplies the sharp, cold, crunchy break that a rich smashed patty and a sweet sauce need to keep from reading as one heavy note. The bun is soft and sized to the patty so it compresses to the meat and absorbs the sauce without competing, and the sauce goes on at assembly so it does not steam the crust soft before it reaches the hand.
The variations are mostly a question of how close the build stays to the codified pairing. Doubling the patty stacks two thin crusts for more seared surface; adding the smashed-onion treatment folds fried onion into the crust and pushes it toward the wider Oklahoma onion burger; dropping the barbecue sauce for a plain dress loosens it back into a generic smash burger entirely. The Oklahoma fried-onion build and the broader American smash burger are the close relatives that share the technique without the fixed Theta dress, and each of those deserves its own article rather than being crowded in here.