Heat the oven to 200c. Spread the sweet potato fries on a baking sheet and toss all over with the cornflour to form a thin layer. Drizzle with oil and season with sea salt and roast for 35 minutes, turning occasionally, until golden and crisp.
Meanwhile, season the meat well, divide ground beef into 4 equal portions, and shape a little into balls. Place the mince patties on the preheated hot pan and, using a spatula, press down to make a shape a little larger than the buns.
Cook for 2-3 minutes, flip and cook for another 3 minutes until cooked through. Do not move or flip the patties multiple times, as tempting as it is, this will affect the cooking process. Add cheese to each patty and cook for another minute until the cheese has melted.