Mix the beef mince, salt and pepper together and press into a lined baking tray. Grill until cooked through.
Toss the grated cheddar in the cornflour and add it and a can of evaporated milk to a saucepan. Heat very gently, stirring all the time, until the cheese has melted into a smooth sauce. Add salt if needed.
Lay tortillas on a similar sized tray to the one you cooked the burger on. Place the burger on top, followed by fried onions , fries and cheese sauce. Fold over the tortillas and squash with another tray then bake until crispy.