Heat the oil in a large lidded pan over a medium heat and fry the onions for 10 mins, stirring frequently until golden. Add the mince, breaking apart any lumps with a wooden spoon. Stir in the peppers and cook for 5 mins.
Add the garlic, thyme and paprika, and stir well. Tip in the tomatoes, tomato purée and stock, and bring to the boil. Reduce the heat and simmer, covered, for 10 mins. Tip in the penne, put the lid back on and bring back to the boil. Reduce the heat again and simmer for another 12 mins until the penne is tender. Leave to stand for 5 mins, then divide half the pasta between two plates and serve. Leave the remainder to cool, then keep chilled for up to four days. Reheat in a pan over a low heat with a splash of water until piping hot.