For the tomato sauce, heat the oil in a pan and fry the onion until soft but not coloured. Add the tomato and tomato purée and stir to combine. Cook for five minutes, stirring occasionally. Add the basil and stock. Blend with a stick blender until smooth.
For the meatballs, mix together the mince, beaten egg, tomato purée and garlic in a bowl. Shape into 4cm/1½in meatballs.
Heat the vegetable oil in a frying pan and fry the meatballs on all sides until golden-brown.
Cook the meatballs in the pan with the tomato sauce on a medium heat for ten minutes, or until they are cooked through (the meat is cooked through when it is tender and has no trace of pink).
For the tacos, brush the tortillas with oil and fry in a frying pan until crisp. Fill the cooked tortillas with the meatballs and tomato sauce and serve.