Lightly whisk the eggs in a bowl.
Finely chop the spring onions (green parts too) and halve the tomatoes.
Heat a spoonful of olive oil in a frying pan. Gently sauté the onion and tomato for 5 minutes.
Pour over the egg mixture, letting it spread out. Arrange the veggies evenly, then scatter most of the cheese on top.
Cook gently for 10 minutes over low heat (under cover).
When the egg has almost set, scatter the remaining cheese all over the top and put the pan under the grill to brown the surface. This should take a couple of minutes.
Transfer the frittata to a plate. Scatter the basil leaves all over, then slice it into wedges or cut it into bites.