So called dentist.

DSCF0947 by geert1953, on Flickr
DSCF0946 by geert1953, on Flickr

The original "dent"was almost the same , so as Fer said. A worker with a big hammer.
You should ask someone to give resistance insite the body of the car, with an other big hammer , or so, to prevent damage to the rest of the body. It is only very thin metal and the flange is the part that gives the most resistance, so the weaker metal around will be inclined to distore more easily than the flange will.

Thank you.

Hey Geert, come over here and bring your big hammer with you to sort my flamer out