Reserve 1 tablespoon of the spinach filling for garnish, and spoon the rest over half the omelette, leaving a 1cm border. Lift up the omelette at the edge nearest the handle of the pan and fold in half, slightly off-centre, so the filling just peeks out. Cook for a further 2 minutes, then slide the omelette onto a plate. Garnish with the reserved filling and serve at once.