1. Preheat your oven to full whack, or get your grill nice and hot. Put the potatoes into a saucepan of boiling salted water and simmer them until cooked, then drain in a colander and leave them to steam dry. Beat the eggs with a fork in a large mixing bowl, season well with salt and pepper, and put to one side.
2. Heat a 20cm non-stick, ovenproof frying pan. Add the chorizo slices and the potato chunks. The chorizo will start to sizzle, releasing all its tasty oils and spices. After a couple of minutes, when everything's lightly golden and crisp, remove from the pan with a slotted spoon and put to one side. Sprinkle the rosemary leaves into the hot fat. As soon as they hit the pan, they'll start to crisp up. Pour the beaten eggs on top immediately, adding the potatoes and chorizo and spreading everything out evenly. Place the whole pan in the preheated oven or under the grill until the omelette is golden brown on top and just cooked through in the middle.
3. While the omelette is cooking, put the shallots into a bowl with the lemon juice, some salt and pepper and a glug of extra virgin olive oil. Toss and pinch the shallots with your fingertips to soften them slightly, then mix in the parsley leaves. Serve a little on top of the omelette.