Boil the ravioli according to package instructions and drain, saving ½ cup of the pasta water. Heat the olive oil in a saucepan over medium heat. Add the onion and fry for 2 minutes. Add the cherry tomatoes, season with salt and pepper and cook for 5 minutes. Reduce the heat, add the pasta water, cover and simmer for another 10 minutes, stirring often until the tomatoes are softened and the sauce has thickened. Pour the sauce over the cooked ravioli, garnish with the basil and serve immediately.