In a bowl, mix all sauce ingredients until smooth. Refrigerate while preparing the burgers.
In another bowl, combine the beef mince with salt, pepper, garlic powder, and smoked paprika.
Divide the beef into four equal patties and flatten slightly.
Heat a pan over high heat and cook patties for about 2 minutes per side until browned.
Toast the brioche buns lightly.
Spread sauce on the bottom bun, then add lettuce, one beef patty, diced onion, and cheese.
Place the extra bun layer in the middle and spread more sauce.
Add pickles, lettuce, another patty, and finish with the top bun.
Repeat to assemble the second burger.