First, prepare the filling. Put the onion in a medium frying pan, and without adding the oil, “sweat” it over medium heat, stirring often, just until slightly softened. Add the oil and when it is heated up, add the tomato paste and pepper paste. Stir to mix. Toss in the rice and sugar, and cook over low heat, stirring frequently to prevent the rice from sticking to the bottom of the pan, for 10-15 minutes. To check for doneness of the filling - take a bite on a rice grain: if it is soft on the outside and only slightly grainy inside, the filling is ready. Take care not to overcook. The rice will cook further inside the peppers. Remove the filling from the heat. Add the mint. Season with salt. Stir to mix and allow to cool.