Preheat BBQ to med-high heat.
In a large mixing bowl, combine beef, egg, breadcrumbs, mustard, onion flakes, salt and pepper. Separate beef into 8 balls, then flatten each as much as possible.
Meanwhile, grill jalapeño for 3-4 min until cooked. Remove from grill, then remove seeds and chop.
Top four of the burgers with cheese and charred jalapeño. Add the other burger on top, sealing all sides carefully. Wet hands with water to finish shaping burgers, taking care to make sure there are no holes for the filling to slip out.
Brush burgers with olive oil on either side, then grill for about 3-5 min per side until beef is cooked to your taste. Toast the buns if you want during the last minute of cooking then serve!