Heat a large cast iron frying pan to medium high heat and add a small amount of oil. Take one large flour tortilla and place it in the pan. Flip the tortilla over a few times, 10 seconds between flips. When pockets of air begin to form within the tortilla remove from the pan and keep to one side.
Chop the feta, tomatoes and olives in small pieces and combine in a large bowl with the sardines and chopped oregano. Mix these well and season with salt and pepper.
Heat another tortilla, as before, leaving it in the pan. Then spoon the sardine and feta mix evenly on the tortilla, pressing down. Add a layer of rocket with a drizzle of olive oil and put the other tortilla on top.
Reduce the heat to low and cover the pan. The cast iron pan should be hot enough by now to have plenty of residual heat to melt the cheese and brown the tortilla. If the quesadilla begins to smoke too much, remove from the heat.
After a minute, use a spatula to lift up one side of the quesadilla and flip over the other side, as if you were making an omelette. The tortilla should by now be browned slightly. If it is not browned, turn the heat up to high and flip the quesadilla over every 10 seconds or so until it gets browned. Remove from pan and cut into wedges. Serve with a green salad.