Wash, drain and dry the rice thoroughly in a clean cloth. Heat the butter in a saucepan, put in the onion, and when lightly browned add the rice, and shake the pan over the heat for about 10 min. Then sprinkle in the saffron, a good pinch of nutmeg, salt and pepper. Cover with stock, and cook gently for about 1 hr., adding meanwhile the tomato sauce and as much stock as the rice will absorb, the sauce being added when the rice is about half cooked. Just before serving stir in the cheese.