Thinly slice the fennel and spring onions.
Heat the olive oil in a non-stick frying pan over a medium-low heat. Add the fennel, cover with a lid and cook gently for about 8 minutes until softened, adding a splash of water if needed.
Stir in the spring onions and cook for another 2 minutes.
Meanwhile, whisk the eggs with a little salt and freshly ground black pepper.
Pour the eggs evenly over the vegetables.
Scatter over the chopped parsley, then arrange the sardine fillets in a circle on top.
Sprinkle with the grated cheese, if using.
Cook gently over a low heat for about 8 minutes, then cover with a lid for another 2–3 minutes until the egg is completely set.
Alternatively, finish the frittata under a hot grill for a couple of minutes until the top is lightly golden.
Leave to rest in the pan for 5 minutes before cutting into wedges, using the sardines as a guide.
Serve warm with a crisp green salad and slices of sourdough or lentil flatbread.