Heat 2 tablespoons oil in a large pan, add the onion and cook for 5 minutes until softened. Stir in 1 chopped garlic clove and the tomato purée and cook for 30 seconds. Stir in the rice and cook for 2 minutes until the rice is well coated.
Pour the wine into the pan and cook until it has bubbled away. Add the saffron, then gradually stir in the hot stock, a little at a time, stirring frequently and allowing the rice to absorb the stock before adding more.
Meanwhile, heat the remaining oil in a large frying pan. Cut the monkfish into large cubes, add to the pan, season and cook for 1–2 minutes on each side until golden. Remove from the pan. Add the remaining stock to the rice along with the fish and cook for a further 5 minutes until the rice and fish are cooked through.
Mix together the remaining chopped garlic with the lemon rind and parsley. Spoon the rice and fish on to plates and scatter over the gremolata to serve.