Heat most of the sunflower oil in a large, heavy-based saucepan over a medium heat. Sweat the mushrooms with a pinch of salt for 6-8 minutes, stirring occasionally. Increase the heat to moderate and continue to cook stirring occasionally until they are browned all over. Transfer to a food processor at this stage and pulse until very finely chopped. Heat the remaining sunflower oil in the same saucepan over a medium heat and sweat the onion and the finely chopped mushrooms for 6-8 minutes until the onion is softened, stirring occasionally. Add the mince and brown all over; 4-5 minutes. Add the passata, tomato sauce, chilli powder, paprika, sugar, Worcestershire sauce and a little seasoning. Bring to a simmer over a medium heat, then reduce to low and simmer for 15 minutes. Adjust the seasoning if necessary, then remove from the heat. Cut the rolls in half and arrange on serving plate, fill generously with the beef mixture. Garnish with the rocket and serve immediately.