Heat the oil in a heavy-based, ovenproof, nonstick 23 cm (9 inch) frying pan over a low heat. Add the onion and cook for 6–8 minutes until softened, then stir in the spinach and peas and cook for a further 2 minutes, or until any moisture released by the spinach has evaporated.
Beat the eggs in a bowl and season lightly with salt and pepper. Stir in the cooked vegetables, then pour the mixture into the pan and quickly arrange the vegetables so that they are evenly dispersed. Cook over a low heat for 8–10 minutes, or until all but the top of the frittata is set.
Transfer the pan to a preheated very hot grill and cook about 10 cm (4 inches) from the heat source until the top is set but not coloured. Give the pan a shake to loosen the frittata, then transfer to a plate to cool. Serve slightly warm or at room temperature, accompanied by a green salad.