Heat half of the oil in a saucepan. Fry the sliced meat until golden brown on all sides. Set aside. Heat the remaining oil in the same pan and fry the onion for 5 minutes, stirring constantly, until tender. Add the paprika and cook for 30 seconds, while still stirring. Add the beef and the tomatoes and their liquid. Bring to a boil, then reduce the heat and cover. Leave to simmer for 10 minutes. Keep warm.