Fry off your onions in the olive oil until they are clear; usually about 4 minutes. Leave to cool for about 5 minutes.
In a bowl add your mince, cumin and Dijon mustard. When the onions are cool add these too. Mix well.
Shape into burgers. Fry on a low/medium heat for around 20 minutes. When nearing the end of cooking, crumble some blue cheese on each burger then put under the grill until melted. Put this into your roll.