Preheat the oven to 180°C/350°F/gas 4. Place the whole leeks straight on to the bars of the oven for 40 minutes, or until blackened. Meanwhile, remove the prawn heads, squashing the juices into a small bowl (sounds gross, but trust me!). Peel away the shells, leaving the tails on, then devein and put aside. Place the prawn shells and heads into a small non-stick pan with 4 tablespoons of olive oil. Cook for 10 minutes on a medium heat, squashing and turning regularly, to create a prawn oil, then discard the shells and heads. Meanwhile, finely slice any larger mushrooms and tear the Pata Negra into small pieces (if needed), then carefully divide and thread the prawns, mushrooms and Pata Negra between 4 skewers (I sometimes like to use long woody rosemary sprigs), not packing them too tightly together.
Strip off and discard the outer leaves from the leeks, then finely chop the rest and place in a medium non-stick frying pan with the beans (juices and all) and the reserved prawn juices. Bring to a simmer, then season to perfection with sea salt and 1 tablespoon of red wine vinegar, adding a splash of water to loosen, if needed. Cook the skewers in a large non-stick frying pan on a medium-high heat (or on a barbecue) with a drizzle of olive oil for 5 minutes, or until golden and just cooked through, turning occasionally and lightly pressing down with a fish slice. Spoon the beans on to a warm serving platter, then place the skewers on top and drizzle with the prawn oil, to serve.