Melt the butter in a saucepan and add the rice. In another pan, mix the milk and the hot water. Stir the rice with a wooden spoon, add a ladleful of warm milk-and-water, stir again. When the liquid has been absorbed by the rice, add another ladleful. Repeat this process until the milk-and-water is used up and the rice is cooked. This will take 18–20 minutes. Add the nutmeg and salt, and mix well.
Heat the oven to 220°C/425°F/Gas Mark 7. Leave the rice to get cool. When it is at room temperature, beat in the egg yolk. Line 4 ramekins with clingfilm and fill them with the rice, pressing it down into each one quite firmly.
Put the ramekins upside-down on a well-greased baking tray or Pyrex dish. Lift off the moulds carefully, and the clingfilm. Scatter the crumbled blue cheese on top of each rice mould, and press gently with a fork. Put the tray in the heated oven and cook for 7–8 minutes, until the cheese bubbles.
While the rice moulds are in the oven, prepare the garnish. Put the tomatoes in a small bowl and pour over boiling water to cover them. Leave for 1 minute, drain, and pour cold water over them; they can now be easily skinned. Cut each skinned tomato into quarters, then each quarter into 2. Spoon out and discard the seeds. Sprinkle the tomatoes with salt and pepper and chives or basil.
Arrange equal portions of tomatoes on 4 small plates. When the rice moulds are ready, put one on each plate and serve immediately.