I've worked with this horse and her pawing for two years and she rarely paws now, but it's something I need to keep an eye on.
In this case she needed to be fed and have reliable food sources. As she was clearly anxious around food, as a result of going hungry.
I also needed to work on my relationship with her and earn her trust. As she was aggressive towards humans, due to being mistreated and was actually frightened of people.
As I earned her trust I was able to start correcting her when she pawed. I would use my foot or a short stick to help me reach across or my hand (depending on where we were both standing) and tap the leg lightly if she looked like she was about to paw or indeed did paw and say "no". It was a painstaking procedure to continually do this, but at that stage it was the only thing I had and I did want to build up a relationship with her, where I could teach her and she would respect me. As she was capable of turning her butt and kicking people or other horses with both barrels. So extremely dangerous when I first got her.
We ended up in a paddock which had electric stand offs on a timber post and rail fence. The ideal fencing set up for horses. So there was a strand of electric fencing wire approximately two feet from the ground, six inches inside the fence line.
I would put her feeder up on the fence so she had to stand at the fence to eat. If she raised her knee to paw, she would occasionally get zapped by the electric fence. It took almost no time to completely eradicate this pawing behaviour when feeding. I was able to feed her away from the fence with success after this, as she grew used to eating without pawing and this experience was without consequence, by that I mean, she learned to eat without pawing and that had no bad consequence - not the electric fence shocking her.
She has only pawed once since this time, recently. There was a lot of competition for the food at the time, a lot of horses in the paddock and she was clearly stressed. So this setback is my fault and our horses are a reflection of us, their owners, as are all our pets. It is by taking this responsibility, I'm able to best serve them as a owner. It can sound harsh, but indeed it is empowering. It means wherever there is a problem, there is a solution.
This little article from Equine Behavior Questions and Answers adequately sums up what is going on with her from a psychological level.
Pawing is often considered a stereotypie – a repetitive behavior that
serves no obvious, outward purpose. However stereotypies do serve a
purpose – they are a coping mechanism that help horses deal with
stress. Some horses start pawing when they eat because they’re nervous
about not getting fed. Sometimes that’s because they were starved
earlier in life and other times it is because they feel threatened
that other horses may steal their food. Once they start pawing, it is
almost impossible to cure them of it – even long after the source of
stress is gone. You can make him feel more secure by making sure he
gets his meal at close to the same time each day and by making sure no
other horses are around to threaten him – either in the same pen as he
is or lunging at him through fences or over stall doors.