Heat the oil in a 20cm (8in) nonstick frying pan. Add the potatoes and season. Cook for 10 minutes, over a medium value heat, until softened and golden.
Preheat the grill to its highest setting. Add the leeks to the frying pan with 1 tbsp water (this will help the veg to steam), then cover and continue cooking for a further 10 minutes.
Stir the roasted peppers and peas into the beaten egg, then season well. Carefully pour the egg mixture into the pan around the potatoes, and cook over a medium heat for 5 minutes, gently moving the egg around the pan to ensure it cooks evenly. Scatter the cheese over the frittata then place the pan under the hot grill and cook until lightly golden. Serve the frittata in thick slices with a fresh, leafy salad and some crusty bread, if you like.