Heat the grill. Heat a tablesp. of the oil in a large non stick pan , add the scallions and half the spinach . Stir-fry for 2-3mins until they soften. Tip into a colander and press down to squeeze out the excess moisture. Beat the eggs and cheese and then add in the spinach mixture & stir well.
Reheat the pan adding the knob of butter, pour in the egg mixture and cook, stirring gently until well set on the base. Finishing cooking under the grill.
Slide onto a large plate for serving or cut into wedges and serve from the pan.
Mix remaining spinach with the olive oil and balsamic vinegar, season lightly and serve with the frittata and some crusty bread.