Lightly whisk the eggs in a mixing bowl. Coarsely grate the cheese and add to the eggs.
Finely chop the spinach, add to the eggs.
Add the chilli, if used and season with salt and pepper.
Heat a spoonful of oil in a non-stick frying pan. Pour in the mixture and let it spread out evenly.
Cook for about 10 minutes, until the surface has almost set.
Turn on the grill in the oven, and put the pan under the grill for a couple of minutes to brown the top of the frittata.
Transfer the frittata to a plate, let it cool slightly, then cut into wedges or squares.