Cook the potatoes in a saucepan of boiling water for 5–6 minutes or until almost tender. Put the spinach in a steamer or colander over the potatoes and cook for another 5 minutes or until the potatoes are tender and the spinach has wilted. Drain the potatoes. Press the spinach with the back of a spoon to extract excess moisture, then chop the leaves.
Heat the oil in a non-stick frying pan that is about 25 cm in diameter. Add the capsicum slices and sauté over a medium heat for 2 minutes. Stir in the drained potatoes and spring onions and continue cooking for 2 minutes.
Beat the eggs in a large bowl, season with salt and pepper and mix in the spinach. With a slotted spoon, remove about half of the vegetables from the pan and add to the egg mixture, leaving the oil in the pan. Stir the egg and vegetables briefly to mix, then pour into the frying pan. Cover and cook, without stirring, for about 6 minutes or until the omelette is almost set but still a little soft on top. Meanwhile, preheat the grill.
Sprinkle the top of the frittata with the parmesan and place under the grill. Cook for 3–4 minutes or until browned and puffed around the edges. Cut into quarters or wedges and serve.