Wash, top and tail the runner beans, then de-string them with a peeler. Cut them into thin strips.
Boil some water in a small saucepan, add the runner beans. Simmer for 5 mins or until just tender. Drain and set aside.
Heat 1 tbsp olive oil in a small non-stick frying pan (23cm diameter). Cook the sliced onions gently until just soft. Remove from pan and set aside.
Whisk the eggs in a bowl. Add the lemon zest, minced garlic, sliced olives. Season with salt and black pepper and mix everything together. Then stir in the cooked beans and onion so they are evenly distributed.
Heat 1 tbsp oil in the frying pan (used for the onions). Pour in the egg mixture and gently stir for a minute or two. Keep on a low-medium heat as it cooks bottom-up, until only the top layer is still liquid. Add a sprinkle of grated parmesan or other strong cheese over the top (optional).
Heat the grill and put the frying pan under it until the top layer of raw egg mixture is cooked and is starting to brown.
Remove from grill. Serve tortilla warm or at room temperature.