Mix the olive oil, salt and freshly ground black pepper in a bowl. We also like to squeeze 1 - 2 cloves of garlic.
Peel the onion and cut into thin rings or narrow wedges. Remove the white skins and seeds from the peppers, then cut into narrow strips. Mix everything together well and let it steep for at least 12 hours.
Stir well before serving and season to taste (if you like, you can refine it with herbs).