Spray a frying pan with low-calorie cooking spray, and set on a medium heat. Add the onions and garlic, then fry for 5 minutes.
Add the courgette and cook for a further 5 minutes — the vegetables should be softened and just turning golden brown.
Add the eggs to a bowl and season to taste with salt and pepper, then whisk with a fork.
Add the fried vegetables, butter beans, and tomatoes. Then stir.
Spray the pan with a little more low-calorie cooking spray and set on a medium heat.
Add the egg and vegetables mixture and cook on a medium heat for 5 minutes — until the eggs have almost set, and the bottom is lightly browned.
Sprinkle the spring onion and grated cheese evenly over the top, then put under a pre-heated grill for 3-5 minutes — until the frittata is set on top but still moist in the middle. Then cut into four and serve.