Combine 150ml wine, the pepper paste, vinegar, paprika and garlic in a jug along with some seasoning. Pour into a large food bag, add the pork steaks and mix together so the pork is completely coated. Push all the air from the bag, seal, and transfer to your fridge. Leave to marinate for at least 5 hours or overnight.
Add 1 tbsp oil to a large frying pan and place over a medium/high heat. Once hot, add the onions and toss in the oil. Turn down the heat and leave the onions to cook for 30-35 minutes until tender and deeply golden. Sir every so often to stop the onions sticking to the bottom of the pan.
Add the remaining wine and use it to deglaze the pan. Once all the wine has been absorbed, add the chilli flakes and continue to cook for a further 1 minute. Scrape the onions into a small bowl and set aside.
Wipe the pan clean and add the remaining oil, leaving it on a medium/high heat. Remove the pork from the marinade, shaking of any excess, and add all the steaks to the pan making sure they are not crowded – they should give out a nice sizzle. Cook for 3 minutes on each side until cooked through and nicely charred. Remove from the pan.
Slice the rolls in half and add the salad leaves, pork, onions and a drizzle of mustard to serve.