In greeting a Muslim I will use my middle finger!
Won’t really bother them. - usually.
If you really want to show offense, take off your shoe and show them the foot. Or the sole of the shoe.
That is a deadly insult and is sure to provoke an instant reaction.
I’ve taught my 4 year old to do this, and it started an uproar at the local Ruby Tuesday’s one evening. I laughed for days.