Bring the water to the boil in a large saucepan. Add salt and pour in the rice. Stir, then cover and lower the heat. Cook gently for around 15 minutes. The rice should be cooked but still retain some bite. Pour it into a sieve and stop the cooking by rinsing under cold running water, drain thoroughly and leave to cool.
Peel the apples and pears, remove the pips and dice the fruit finely. Sprinkle with lemon juice.
Make the vinaigrette: place the wholegrain mustard in a bowl, incorporate the vinegar and a little salt, whisk and add the 2 oils drop by drop, stirring all the time. Add pepper.
Arrange the rice, diced apples and pears and finally the shelled walnuts in a large bowl. Top with the grated lemon rind and the vinaigrette.