Thinly slice the leek. Using a bean slicer or mandoline, slice the runner beans into long, thin strips.
Lightly beat the eggs in a bowl. Stir in most of the grated halloumi and the dried oregano, reserving a small handful of the cheese for the topping.
Heat a little olive oil in a non-stick frying pan over a low to medium heat.
Add the leek and runner beans and cook gently for about 8 minutes until softened, adding a splash of water if needed to prevent them from catching.
Pour the egg mixture evenly over the vegetables.
When the frittata is almost set but still slightly soft on top, scatter over the remaining halloumi.
Cover the pan with a lid and cook for a further few minutes until the eggs are fully set and the cheese has melted.
If you prefer a golden, lightly crisp top, place the pan under a hot grill for 2–3 minutes instead of covering with a lid.
Slice into wedges and serve warm with wild rocket, sourdough and a green salad.